diff --git a/moshi/src/main/java/com/squareup/moshi/JsonAdapter.java b/moshi/src/main/java/com/squareup/moshi/JsonAdapter.java
index 9143b16..a35fc96 100644
--- a/moshi/src/main/java/com/squareup/moshi/JsonAdapter.java
+++ b/moshi/src/main/java/com/squareup/moshi/JsonAdapter.java
@@ -130,6 +130,9 @@ public abstract class JsonAdapter<T> {
    * nulls.
    */
   @CheckReturnValue public final JsonAdapter<T> nullSafe() {
+    if (this instanceof NullSafeJsonAdapter) {
+      return this;
+    }
     return new NullSafeJsonAdapter<>(this);
   }
 
@@ -141,6 +144,9 @@ public abstract class JsonAdapter<T> {
    * handled elsewhere. This should only be used to fail on explicit nulls.
    */
   @CheckReturnValue public final JsonAdapter<T> nonNull() {
+    if (this instanceof NonNullJsonAdapter) {
+      return this;
+    }
     return new NonNullJsonAdapter<>(this);
   }
 
diff --git a/moshi/src/test/java/com/squareup/moshi/JsonAdapterTest.java b/moshi/src/test/java/com/squareup/moshi/JsonAdapterTest.java
index 590e283..2fbfa6a 100644
--- a/moshi/src/test/java/com/squareup/moshi/JsonAdapterTest.java
+++ b/moshi/src/test/java/com/squareup/moshi/JsonAdapterTest.java
@@ -285,4 +285,14 @@ public final class JsonAdapterTest {
     }.lenient().serializeNulls();
     assertThat(adapter.fromJson("true true")).isEqualTo(true);
   }
+
+  @Test public void nullSafeDoesntDuplicate() {
+    JsonAdapter<Boolean> adapter = new Moshi.Builder().build().adapter(Boolean.class).nullSafe();
+    assertThat(adapter.nullSafe()).isSameAs(adapter);
+  }
+
+  @Test public void nonNullDoesntDuplicate() {
+    JsonAdapter<Boolean> adapter = new Moshi.Builder().build().adapter(Boolean.class).nonNull();
+    assertThat(adapter.nonNull()).isSameAs(adapter);
+  }
 }