Nothing goes wrong. Each bill can only be counted once, so scanning it again does not add anything and does not cause a problem.
If you scan a bill you have already scanned, the app will simply tell you it has been used.
Check the list of credits in the Cashback section. Two credits from the same shopping trip usually means two different offers both paid — which is normal, not a duplicate.
If you genuinely see the same offer credited twice for one bill, get in touch with the date and the shop name.
Still stuck? Reply to any ZlipR notification or write to us from the app.