If you work full time in Spain for a company, but you have an online business that is part time self employed work; do you have to pay the self employed 250 euros a month fee?

If I had to take an educated guess I'd say no, as it's really the equivalent of NI contributions so it would mean you'd be paying those twice...however, this is Spain so you never know. It may be worth asking someone from the Pay/accountancy dept where you work full time.
This link was posted on another thread(by Domino), there may be something in there that will be of help to you, hopefully...
http://www.advoco.es/hot-topics/43-g...system.html#A2

We have family members here who work full time for seperate companies. They also run their own business on a self employed basis. They have to pay all self employed taxes and the 250 social security payment etc. so yes they do pay twice.

Oh well, I think I need to work on my "educated guesswork"

presumably a 65yo retiree would stil have to register as autonomo to get the IBA number for invoices - but would they have to pay the €260 per month ? ?