It could well be the mobile app.

I’ve never been able to get my banks cheque depositing app to work, it always throws one issue or another and then they say I need to get another one issued.
Then when I take the ‘rejected’ cheque to a branch I can deposit it with no problem.
Personally I’ve come to the conclusion that the apps are just window dressing and hardly ever work.
Looking online I’m not the only person to reach that conclusion.

Thank you. Might just try and deposit in branch first 

Please come back and let us know if the branch accepts it without a problem.

EDIT
I just had another thought. Maybe your app has hiccuped and submitted it twice?

So the first one had been accepted, but the second one sent microseconds later rejected as a duplicate?

I’d check your account to see if one has already gone in there.

Hey @Nolpecker 

Did you get that problem with the duplication sorted out?

All sorted thanks. Branch accepted it no problem.  Seems to be an issue with the banking app (Santander) and it's ability to read cheques that are totally blank on one side. 

Yeah, those mobile cheque scanning features are terrible - you get at best a 50% success rate depending on which app you’re using.

Some are better than others...