diff --git a/test/output.go b/test/output.go
index d2efea70..95e652b4 100644
--- a/test/output.go
+++ b/test/output.go
@@ -19,6 +19,7 @@ package test
 
 import (
 	"bytes"
+	"encoding/hex"
 	"encoding/json"
 	"errors"
 	"flag"
@@ -424,6 +425,32 @@ I output.go:<LINE>] "test" firstKey=1 secondKey=3
 		text:   "cycle",
 		values: []interface{}{"list", newCyclicList()},
 		expectedOutput: `I output.go:<LINE>] "cycle" list="<internal error: json: unsupported value: encountered a cycle via *test.myList>"
+`,
+	},
+	"duplicates": {
+		withValues:     []interface{}{"trace", traceIDFromHex("101112131415161718191A1B1C1D1E1F"), "span", spanIDFromHex("0102030405060708")},
+		moreValues:     []interface{}{"trace", traceIDFromHex("101112131415161718191A1B1C1D1E1F"), "span", spanIDFromHex("1112131415161718")},
+		evenMoreValues: []interface{}{"trace", traceIDFromHex("101112131415161718191A1B1C1D1E1F"), "span", spanIDFromHex("2122232425262728")},
+		text:           "duplicates",
+
+		// No de-duplication among WithValues calls at the moment!
+		expectedOutput: `I output.go:<LINE>] "duplicates" trace="101112131415161718191a1b1c1d1e1f" span="0102030405060708"
+I output.go:<LINE>] "duplicates" trace="101112131415161718191a1b1c1d1e1f" span="0102030405060708" trace="101112131415161718191a1b1c1d1e1f" span="1112131415161718"
+I output.go:<LINE>] "duplicates" trace="101112131415161718191a1b1c1d1e1f" span="0102030405060708"
+I output.go:<LINE>] "duplicates" trace="101112131415161718191a1b1c1d1e1f" span="0102030405060708" trace="101112131415161718191a1b1c1d1e1f" span="2122232425262728"
+`,
+	},
+	"mixed duplicates": {
+		withValues:     []interface{}{"trace", traceIDFromHex("101112131415161718191A1B1C1D1E1F"), "span", spanIDFromHex("0102030405060708"), "a", 1},
+		moreValues:     []interface{}{"b", 2, "trace", traceIDFromHex("101112131415161718191A1B1C1D1E1F"), "span", spanIDFromHex("1112131415161718")},
+		evenMoreValues: []interface{}{"c", 3, "trace", traceIDFromHex("101112131415161718191A1B1C1D1E1F"), "span", spanIDFromHex("2122232425262728"), "d", 4},
+		text:           "duplicates",
+
+		// No de-duplication among WithValues calls at the moment!
+		expectedOutput: `I output.go:<LINE>] "duplicates" trace="101112131415161718191a1b1c1d1e1f" span="0102030405060708" a=1
+I output.go:<LINE>] "duplicates" trace="101112131415161718191a1b1c1d1e1f" span="0102030405060708" a=1 b=2 trace="101112131415161718191a1b1c1d1e1f" span="1112131415161718"
+I output.go:<LINE>] "duplicates" trace="101112131415161718191a1b1c1d1e1f" span="0102030405060708" a=1
+I output.go:<LINE>] "duplicates" trace="101112131415161718191a1b1c1d1e1f" span="0102030405060708" a=1 c=3 trace="101112131415161718191a1b1c1d1e1f" span="2122232425262728" d=4
 `,
 	},
 }
@@ -1022,3 +1049,65 @@ func newCyclicList() *myList {
 	a.Next = b
 	return a
 }
+
+// SpanID mimicks https://pkg.go.dev/go.opentelemetry.io/otel/trace#SpanID.
+type SpanID [8]byte
+
+var (
+	_ json.Marshaler = SpanID{}
+	_ fmt.Stringer   = SpanID{}
+)
+
+func (s SpanID) MarshalJSON() ([]byte, error) {
+	return json.Marshal(s.String())
+}
+
+func (s SpanID) String() string {
+	return hex.EncodeToString(s[:])
+}
+
+func spanIDFromHex(str string) SpanID {
+	decoded, err := hex.DecodeString(str)
+	if err != nil {
+		panic(fmt.Sprintf("invalid hex string %q: %v", str, err))
+	}
+	if len(decoded) != len(SpanID{}) {
+		panic(fmt.Sprintf("invalid length of hex string %q: need %d bytes", str, len(SpanID{})))
+	}
+	var result SpanID
+	for i := range result {
+		result[i] = decoded[i]
+	}
+	return result
+}
+
+// TraceID mimicks https://pkg.go.dev/go.opentelemetry.io/otel/trace#TraceID.
+type TraceID [16]byte
+
+var (
+	_ json.Marshaler = TraceID{}
+	_ fmt.Stringer   = TraceID{}
+)
+
+func (s TraceID) MarshalJSON() ([]byte, error) {
+	return json.Marshal(s.String())
+}
+
+func (s TraceID) String() string {
+	return hex.EncodeToString(s[:])
+}
+
+func traceIDFromHex(str string) TraceID {
+	decoded, err := hex.DecodeString(str)
+	if err != nil {
+		panic(fmt.Sprintf("invalid hex string %q: %v", str, err))
+	}
+	if len(decoded) != len(TraceID{}) {
+		panic(fmt.Sprintf("invalid length of hex string %q: need %d bytes", str, len(TraceID{})))
+	}
+	var result TraceID
+	for i := range result {
+		result[i] = decoded[i]
+	}
+	return result
+}
diff --git a/test/zapr.go b/test/zapr.go
index 96f175f4..be766c72 100644
--- a/test/zapr.go
+++ b/test/zapr.go
@@ -258,6 +258,26 @@ I output.go:<LINE>] "odd WithValues" keyWithoutValue="(MISSING)"
 
 		`I output.go:<LINE>] "cycle" list="<internal error: json: unsupported value: encountered a cycle via *test.myList>"
 `: `{"caller":"test/output.go:<LINE>","msg":"cycle","v":0,"listError":"json: unsupported value: encountered a cycle via *test.myList"}
+`,
+
+		`I output.go:<LINE>] "duplicates" trace="101112131415161718191a1b1c1d1e1f" span="0102030405060708"
+I output.go:<LINE>] "duplicates" trace="101112131415161718191a1b1c1d1e1f" span="0102030405060708" trace="101112131415161718191a1b1c1d1e1f" span="1112131415161718"
+I output.go:<LINE>] "duplicates" trace="101112131415161718191a1b1c1d1e1f" span="0102030405060708"
+I output.go:<LINE>] "duplicates" trace="101112131415161718191a1b1c1d1e1f" span="0102030405060708" trace="101112131415161718191a1b1c1d1e1f" span="2122232425262728"
+`: `{"caller":"test/output.go:<LINE>","msg":"duplicates","trace":"101112131415161718191a1b1c1d1e1f","span":"0102030405060708","v":0}
+{"caller":"test/output.go:<LINE>","msg":"duplicates","trace":"101112131415161718191a1b1c1d1e1f","span":"0102030405060708","trace":"101112131415161718191a1b1c1d1e1f","span":"1112131415161718","v":0}
+{"caller":"test/output.go:<LINE>","msg":"duplicates","trace":"101112131415161718191a1b1c1d1e1f","span":"0102030405060708","v":0}
+{"caller":"test/output.go:<LINE>","msg":"duplicates","trace":"101112131415161718191a1b1c1d1e1f","span":"0102030405060708","trace":"101112131415161718191a1b1c1d1e1f","span":"2122232425262728","v":0}
+`,
+
+		`I output.go:<LINE>] "duplicates" trace="101112131415161718191a1b1c1d1e1f" span="0102030405060708" a=1
+I output.go:<LINE>] "duplicates" trace="101112131415161718191a1b1c1d1e1f" span="0102030405060708" a=1 b=2 trace="101112131415161718191a1b1c1d1e1f" span="1112131415161718"
+I output.go:<LINE>] "duplicates" trace="101112131415161718191a1b1c1d1e1f" span="0102030405060708" a=1
+I output.go:<LINE>] "duplicates" trace="101112131415161718191a1b1c1d1e1f" span="0102030405060708" a=1 c=3 trace="101112131415161718191a1b1c1d1e1f" span="2122232425262728" d=4
+`: `{"caller":"test/output.go:<LINE>","msg":"duplicates","trace":"101112131415161718191a1b1c1d1e1f","span":"0102030405060708","a":1,"v":0}
+{"caller":"test/output.go:<LINE>","msg":"duplicates","trace":"101112131415161718191a1b1c1d1e1f","span":"0102030405060708","a":1,"b":2,"trace":"101112131415161718191a1b1c1d1e1f","span":"1112131415161718","v":0}
+{"caller":"test/output.go:<LINE>","msg":"duplicates","trace":"101112131415161718191a1b1c1d1e1f","span":"0102030405060708","a":1,"v":0}
+{"caller":"test/output.go:<LINE>","msg":"duplicates","trace":"101112131415161718191a1b1c1d1e1f","span":"0102030405060708","a":1,"c":3,"trace":"101112131415161718191a1b1c1d1e1f","span":"2122232425262728","d":4,"v":0}
 `,
 	}
 }
@@ -345,6 +365,26 @@ I output.go:<LINE>] "test" firstKey=1 secondKey=3
 		// discards these messages.
 		`I output.go:<LINE>] "v=11: you see me because of -vmodule output=11"
 `: ``,
+
+		`I output.go:<LINE>] "duplicates" trace="101112131415161718191a1b1c1d1e1f" span="0102030405060708"
+I output.go:<LINE>] "duplicates" trace="101112131415161718191a1b1c1d1e1f" span="0102030405060708" trace="101112131415161718191a1b1c1d1e1f" span="1112131415161718"
+I output.go:<LINE>] "duplicates" trace="101112131415161718191a1b1c1d1e1f" span="0102030405060708"
+I output.go:<LINE>] "duplicates" trace="101112131415161718191a1b1c1d1e1f" span="0102030405060708" trace="101112131415161718191a1b1c1d1e1f" span="2122232425262728"
+`: `{"caller":"test/output.go:<LINE>","msg":"duplicates","v":0,"trace":"101112131415161718191a1b1c1d1e1f","span":"0102030405060708"}
+{"caller":"test/output.go:<LINE>","msg":"duplicates","v":0,"trace":"101112131415161718191a1b1c1d1e1f","span":"0102030405060708","trace":"101112131415161718191a1b1c1d1e1f","span":"1112131415161718"}
+{"caller":"test/output.go:<LINE>","msg":"duplicates","v":0,"trace":"101112131415161718191a1b1c1d1e1f","span":"0102030405060708"}
+{"caller":"test/output.go:<LINE>","msg":"duplicates","v":0,"trace":"101112131415161718191a1b1c1d1e1f","span":"0102030405060708","trace":"101112131415161718191a1b1c1d1e1f","span":"2122232425262728"}
+`,
+
+		`I output.go:<LINE>] "duplicates" trace="101112131415161718191a1b1c1d1e1f" span="0102030405060708" a=1
+I output.go:<LINE>] "duplicates" trace="101112131415161718191a1b1c1d1e1f" span="0102030405060708" a=1 b=2 trace="101112131415161718191a1b1c1d1e1f" span="1112131415161718"
+I output.go:<LINE>] "duplicates" trace="101112131415161718191a1b1c1d1e1f" span="0102030405060708" a=1
+I output.go:<LINE>] "duplicates" trace="101112131415161718191a1b1c1d1e1f" span="0102030405060708" a=1 c=3 trace="101112131415161718191a1b1c1d1e1f" span="2122232425262728" d=4
+`: `{"caller":"test/output.go:<LINE>","msg":"duplicates","v":0,"trace":"101112131415161718191a1b1c1d1e1f","span":"0102030405060708","a":1}
+{"caller":"test/output.go:<LINE>","msg":"duplicates","v":0,"trace":"101112131415161718191a1b1c1d1e1f","span":"0102030405060708","a":1,"b":2,"trace":"101112131415161718191a1b1c1d1e1f","span":"1112131415161718"}
+{"caller":"test/output.go:<LINE>","msg":"duplicates","v":0,"trace":"101112131415161718191a1b1c1d1e1f","span":"0102030405060708","a":1}
+{"caller":"test/output.go:<LINE>","msg":"duplicates","v":0,"trace":"101112131415161718191a1b1c1d1e1f","span":"0102030405060708","a":1,"c":3,"trace":"101112131415161718191a1b1c1d1e1f","span":"2122232425262728","d":4}
+`,
 	} {
 		mapping[key] = value
 	}