How to prevent from adding tag that already exist?

//create global variable
private List<String> mTagList = new ArrayList<>();

//add tags change listener to TagsEdittext
mItemTags.setTagsListener(new TagsEditText.TagsEditListener() {
            @Override
            public void onTagsChanged(Collection<String> collection) {
                Timber.i("onTagsChanged : %s", collection);
                if (mTagList.size() != mItemTags.getTags().size()) {
                    boolean isDataChange = false;
                    mTagList.clear();
                    for (int index = 0; index < mItemTags.getTags().size(); index++) {
                        if (!mTagList.contains(mItemTags.getTags().get(index))) {
                            isDataChange = true;
                            mTagList.add(mItemTags.getTags().get(index));
                        }
                    }
                    if (isDataChange) {
                        String[] tmp = new String[mTagList.size()];
                        for (int index = 0; index < tmp.length; index++) {
                            tmp[index] = mTagList.get(index);
                        }
                        mItemTags.setTags(tmp);
                    }
                }
            }

            @Override
            public void onEditingFinished() {
                Timber.i("onEditingFinished");
            }
        });