src/main/java/org/fest/assertions/ItemGroupAssert.java
protected final void assertDoesNotHaveDuplicates() {
    isNotNull();
    Collection<?> duplicates = duplicatesFrom(actualAsList());
    if (duplicates.isEmpty()) {
      return;
    }
    failIfCustomMessageIsSet();
    throw failure(format("<%s> contains duplicate(s):<%s>", actual, duplicates));
  }
protected final void assertDoesNotHaveDuplicates() {
    isNotNull();
    Collection<?> duplicates = duplicatesFrom(actualAsList());
    if (duplicates.isEmpty()) {
      return;
    }
    failIfCustomMessageIsSet();
    throw failure(format("<%s> contains duplicate(s):<%s>", actual, duplicates));
  }
Verifies that the actual group of objects does not have duplicates.

@throws AssertionError if the actual group of objects is {@code null}.
@throws AssertionError if the actual group of objects has duplicates.