Filling now gone dark one front tooth

I would not rebond these teeth as It looks to me like they have had all of the composite that they can take.  You will get a much better result by placing full porcelain crowns on these teeth.  I would have this procedure performed by a dentist with a great deal of experience doing cosmetic dentistry.  If not performed correctly these crowns will have a slightly gray appearance.  The underlying tooth must be blocked out with a very white composite material before placing the crowns.  Good luck.  

I would suggest not redoing the bonding as the staining will continue to reform and contant re-bonding  can eventually weaken the tooth further. The bonding looks to be replacing a large part of that front tooth.  A better aesthetic and protective option would be a full ceramic crown over your right central incisor.  This in combination with a crown or porcelain veneer on the left central incisor would give the ability to match the shade and texture in the most harmonious way.of course an exam and radiograph of the front two teeth would be needed before knowing if this aesthetic option can work for you.

It's possible to bond that tooth again to try for a better color match, but there is such a large composite filling there now that I don't think it's worth it long term. More bonding might work for a while, but at some point I think that tooth will need a crown.

It appears that the filling is leaking,ie that it didn't bond properly. I would assume that this has been replaced more than once and it is getting larger and not lasting as long each time. It's time to look into doing something more permanent such as a crown. Looks like too much tooth gone for a veneer but the dentist could let you know on that.