There is no problem in attending Sunni congregational prayers; it is permissible and there are some reports according to which one who stands in the first row of their congregational prayers is like one who has offered his prayers standing behind the Holy Prophet (S) in the first row.[1]
In any case, if possible, one should offer his prayers according to Shia way and if there is no need for Taqiyah (dissimulation) and one offers his prayers in Sunni way, he must repeat his prayers.  Of course, there are some scholars[2] who are of the view that if the conditions for practicing Taqiyah are not met, one must repeat his prayers under all circumstances.
Index:
Some grand religious authorities have also replied to the same question. Their answers are as follows: [3]
Grand Ayatollah Khamenei (may Allah grant him long life):
In the said case, if using turbah is not in opposition with practicing Taqiyah, it is necessary to do Sajda on something which is allowed for prostration.
Grand Ayatollah Makarem Shirazi (may Allah grant him long life):
If you are associating with them, there is no problem in offering prayers with them to protect yourself provided that they do not express enmity against Shia. However, doing Sajda on carpet is not allowed except where one practices Taqiyah in Sunni mosques.
Grand Ayatollah Noori Hamedani (may Allah grant him long life):
If possible, one should do Sajda on things which are allowable for Sajdah but there is no problem in offering prayers behind a Sunni prayer leader and one is not, in the said case, required to repeat his prayers.
Grand Ayatollah Saafi Gulpaigani (may Allah grant him long life):
If you are not in the state of Taqiyah, it is not sufficient and it would be necessary to repeat the prayers.

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