For Android users, you can also try to tap the "Remove Watermark and Ads" button again. If your device is connected to the same Google Play account, it should directly unlock the feature without charging you again.
For iOS users, you can also try:
1. Tap the "Done" icon on collage editing page
2. Tap "Remove Watermark and Ads" button
3. If the previous purchase has been successful, there should be a pop-up stating "You've already purchased this. Would you like to get it again for free?" After confirming purchase, please tap "OK" and the watermark should disappear.
Apple won't charge you again by using the same iTunes account to purchase the same products you've paid before.
Please refer to the instruction collage below: