How to combine 2 IDs

Hi guys

I am not a person wo knows a lot about computers, so bear with me :-).

Apparantly I have 2 IDs, one probably created with another Email address a long time ago. Now I wanted to re-read an old book again and wanted to download it into my DE library from the place I bought it from and was told, that that E-Lic was already fulfilled by another user. There is no one else in my household using DE so it is me :-).

Now my first question is:

How can I find out what my other ID is if I don't even remember what Email I might have used?

How can I combine the two IDs so I have access to all the books?

Appreciate all help on this, thank you!