I have accepted a postdoc offer, then an other offer came out and I declined it mostly because I already accepted the first one. After taking time to think and consider multiple factors I have some regrets. Is it normal to ask if that position has been filled yet and if not is it okay to take it?
Proceed with caution here. Regret is common and, as we say "The grass is always greener on the other side of the fence".
I can't really recommend one way or the other, since it is both your decision and you who will benefit or suffer from any decision you make. But I suggest you consider the following.
You leave yourself at risk of seeming unreliable to both institutions if you try to make a change after you have accepted an offer. That wouldn't serve you well if it were to occur.
Personally, I would only be tempted to try to switch if there were something decidedly negative about the accepted offer/position. I would be more inclined to write the second institution, thanking them, telling them that you just recently accepted another offer before this one was made and that perhaps the future will offer possibilities for joining them.
But, consider that there might be risk in trying for a short term optimization here. Think about the longer term.