Can a person have serotonin syndrome after taking an ssri pill twice?

Not very likely: Ssri's are fairly safe medications. While it is never a good idea to take more medication than has been prescribed to you, you should be ok if you accidentally took two pills in the same day. If you feel feverish, are nauseated or are vomiting, go to the emergency room. Fevers in serotonin syndrome are quite high, and they are usually accompanied by delirium and confusion - so it is very unlikely.

It's possible: One could get serotonin syndrome after only one dose of an ssri. Rare but possible.