Can the dns' domain be in its own spf record?

I'm finally fixing up my dns zone files in Gandi, which has a handy feature that lets me use the same zone file for several domains. So I can keep 1 zone file for a single domain and all its tld's.

But normally, when you set up the SPF record for eg domain.com, you don't actually add "mx:domain.com", because it's implied.

Will this thrown an error if you do?

Here's an example SPF record for domain.com.

The "correct" one:

v=spf1 mx mx:domain.info mx:domain.net mx:domain.org ~all

And here's the one I would like:

v=spf1 mx mx:domain.info mx:domain.net mx:domain.org mx:domain.com ~all

Is that allowed, or not?

It's not implied. There's nothing implicitly assumed about what systems you want to authorize.

You're explicitly setting mx:domain.com by setting mx at the start of the record - they're functionally identical when the SPF entry is on domain.com.

That said, you should not run into any validation errors simply because you specify the same thing twice.