diff --git a/interfaceinjection/src/main/java/net/neoforged/jst/interfaceinjection/InjectInterfacesVisitor.java b/interfaceinjection/src/main/java/net/neoforged/jst/interfaceinjection/InjectInterfacesVisitor.java
index f129240..314fc07 100644
--- a/interfaceinjection/src/main/java/net/neoforged/jst/interfaceinjection/InjectInterfacesVisitor.java
+++ b/interfaceinjection/src/main/java/net/neoforged/jst/interfaceinjection/InjectInterfacesVisitor.java
@@ -79,6 +79,10 @@ private void inject(PsiClass psiClass, Collection<String> targets) {
                 .sorted(Comparator.naturalOrder())
                 .collect(Collectors.joining(", "));
 
+        if (interfaceImplementation.isEmpty()) {
+            return; // Nothing to do, all injected interfaces are already present
+        }
+
         if (implementsList.getChildren().length == 0) {
             StringBuilder text = new StringBuilder();
 
diff --git a/tests/data/interfaceinjection/deduplication/expected/pkg/Example.java b/tests/data/interfaceinjection/deduplication/expected/pkg/Example.java
new file mode 100644
index 0000000..2b0642d
--- /dev/null
+++ b/tests/data/interfaceinjection/deduplication/expected/pkg/Example.java
@@ -0,0 +1,6 @@
+package pkg;
+
+import java.util.Set;
+
+public class Example implements Set {
+}
diff --git a/tests/data/interfaceinjection/deduplication/injectedinterfaces.json b/tests/data/interfaceinjection/deduplication/injectedinterfaces.json
new file mode 100644
index 0000000..b9b77c7
--- /dev/null
+++ b/tests/data/interfaceinjection/deduplication/injectedinterfaces.json
@@ -0,0 +1,3 @@
+{
+  "pkg/Example": "java/util/Set"
+}
diff --git a/tests/data/interfaceinjection/deduplication/source/pkg/Example.java b/tests/data/interfaceinjection/deduplication/source/pkg/Example.java
new file mode 100644
index 0000000..2b0642d
--- /dev/null
+++ b/tests/data/interfaceinjection/deduplication/source/pkg/Example.java
@@ -0,0 +1,6 @@
+package pkg;
+
+import java.util.Set;
+
+public class Example implements Set {
+}
diff --git a/tests/src/test/java/net/neoforged/jst/tests/EmbeddedTest.java b/tests/src/test/java/net/neoforged/jst/tests/EmbeddedTest.java
index 34372f8..3f195b8 100644
--- a/tests/src/test/java/net/neoforged/jst/tests/EmbeddedTest.java
+++ b/tests/src/test/java/net/neoforged/jst/tests/EmbeddedTest.java
@@ -316,6 +316,11 @@ void testSimpleInjection() throws Exception {
             runInterfaceInjectionTest("simple_injection", tempDir);
         }
 
+        @Test
+        void testDeduplication() throws Exception {
+            runInterfaceInjectionTest("deduplication", tempDir);
+        }
+
         @Test
         void testAdditiveInjection() throws Exception {
             runInterfaceInjectionTest("additive_injection", tempDir);