Can I set up a different signature for each of my Gmail aliases?

Yes – you can do that. Here is how to design email signatures for your Gmail aliases is explained in the support note with name How to set up email signatures for aliases.

Can I have multiple signatures for the same Gmail email address?

Yes, you can do that. How to design email signatures for your Gmail aliases is explained in the support note named How to set up email signatures for aliases.

Can I have the same signature on mobile?

Yes – you can do that. You will need to have the Mobile Signature option unchecked in your settings, and the Gmail App or mobile web mail interface will automatically pick up your desktop signature. You can read more in the support note titled How to set an email signature on mobile (Gmail mobile app).

I'm seeing double signatures. How to fix that?

To ensure that signatures are not added to replies or forwards, you need to disable the option Insert signature on reply/forward. This option controls whether the signature is added in a reply or not. You can read more in the support note titled How to control whether the signature is added on a reply or forward.