Fix TraceStateBuilder.remove double-counting a repeated removal

api/all/src/main/java/io/opentelemetry/api/trace/ArrayBasedTraceStateBuilder.java

@@ -92,8 +92,13 @@ public TraceStateBuilder remove(String key) {
     }
     for (int i = 0; i < reversedEntries.size(); i += 2) {
       if (reversedEntries.get(i).equals(key)) {
-        reversedEntries.set(i + 1, null);
-        numEntries--;
+        // Only account for the removal if the entry is still present. A repeated remove of an
+        // already-removed key must be a no-op, mirroring the guard in put(); otherwise numEntries
+        // is decremented twice and build() drops unrelated entries or emits a null-valued entry.
+        if (reversedEntries.get(i + 1) != null) {
+          reversedEntries.set(i + 1, null);
+          numEntries--;
+        }
         return this;
       }
     }

api/all/src/test/java/io/opentelemetry/api/trace/TraceStateTest.java

@@ -303,6 +303,40 @@ void removeNotPresent() {
         .isEqualTo(multiValueTraceState);
   }
 
+  @Test
+  void removeAlreadyRemovedKeyKeepsOtherEntries() {
+    TraceState state =
+        TraceState.builder()
+            .put("a", "1")
+            .put("b", "2")
+            .remove("a")
+            .remove("a") // removing an already-removed key must be a no-op
+            .build();
+    assertThat(state.get("b")).isEqualTo("2");
+    assertThat(state.size()).isEqualTo(1);
+  }
+
+  @Test
+  void removeAlreadyRemovedKeyDoesNotProduceNullValue() {
+    TraceState state = TraceState.builder().put("a", "1").remove("a").remove("a").build();
+    assertThat(state.isEmpty()).isTrue();
+    assertThat(state.get("a")).isNull();
+  }
+
+  @Test
+  void removeAlreadyRemovedKeyWithMultipleEntriesDoesNotThrow() {
+    assertThatCode(
+            () ->
+                TraceState.builder()
+                    .put("a", "1")
+                    .put("b", "2")
+                    .put("c", "3")
+                    .remove("a")
+                    .remove("a")
+                    .build())
+        .doesNotThrowAnyException();
+  }
+
   @Test
   void addAndRemoveEntry() {
     assertThat(