I do not understand why the agent cannot write and deliver a second get where there is a doubt as to whether the writing and/or delivery of the first get were valid. I would think that if the first get were valid, the second get would be unauthorized and invalid and unnecessary but harmless and that if the first get were invalid, the shelichut would not have terminated and the second get would be authorized and consequently valid. I do not see why the second get cannot be given 'al tnai so that the woman would surely be divorced either by the first or the second get.
Michael S. Winokur, Forest Hills, New York, U.S.A.
(1) It seems to me that your question can be answered on the basis of what the Rosh writes here that the reason we are uncertain if the Get has been written properly or not, is because of various Chumros that the custom has developed to be stringent for. The Rosh writes that these stringencies have no root or basis in Torah law.
Therefore it appears that according to pure Halachah the Get is in reality valid. Consequently the witnesses have performed their Shelichus according to the Halachah. However, we have various customs and stringent practices and therefore wish to write the Get again, but the witnesses do not have the power to do this because in reality they got it write the first time, so they have already finished their job.
In other words there is not a genuine doubt that the first Get might not be valid, so therefore it is not appropriate to write the second Get Al Tenai.
(2) Another answer can be found in the Pischei Teshuvah Shulchan Aruch EH 121:5, based on the Shach CM 49:6 that even if the get is not doubtful because of Chumros but rather because of a possible d'Oraisa deficiency, nevertheless, since the Get is not definitely invalid, one still says that the witnesses have performed their Shelichus. The Pischei Teshuvah cites the Toras Gitin who explains the Shach that the sort of doubt being referred to is where the leading scholar in town is not able to decide the question, but a greater sage might not be in doubt at all about it. Therefore in such a case one says that this is not a genuine doubt because the root of our doubt is simply our lack of expertise.
Again, in such a case the first Get is not really invalid, so the witnesses have in fact terminated their Shelichus, and cannot write a second Get and in addition it is not appropriate to write a second Get Al Tenai.
Kesivah u'Chasimah Tovah
Dovid Bloom