When placing a bet, it appears that when pressing the place bet/confirm bet for an in-play market that it is unclear whether your bet has been submitted. Unlike when using a browser and seeing the countdown, there is no confirmation that you have pressed the place bet/confirm. I have found that if you press the confirm bet more than once, it will resubmit your original bet multiple times.
Once a bet has been submitted it, the app should not allow you to press the submit button again until you know your original bet has been correctly submitted to the market.
The same happens for markets that are not in-play when there is site lag or the app runs slow. It can appear that your bet hasn't gone through when it actually has but takes a few secs to appear. This can lead you to mistakenly resubmitting a bet if it doesn't look like it went through the first time.