root canal twice in one tooth?????please help!

Hi... i went to the dentist recently because i have a tooth that i had a root canal on a while back and it was changing colors... and starting to fall apart... she xrayed it and sent me to a root canal specialist saying she didn't want to cap and post it with infection in there... doesn't an infection cause pain???... they were gracious enough to let me mailed my xrays to them so they could tell me whether or not i need another root canal in the same tooth... they said i did... the tooth doesn't hurt... it's just falling apart and now the filling has fallen out so i must deal with this soon...i'm confused about why i need another root canal in a tooth that has already had one... and it doesn't hurt or anything... what are my options... can someone please help me... i need to do something soon... thank you
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