How can there be a case where one is obligated to say the berachah of shehechayanu twice on the mitzvah of lulav and succah over Succos?
The Rambam (Hilchos Melochim 10:10) rules, if a non-Jew wants to fulfill a mitzvah in order to get reward we don’t stop him, and we let him fulfill the mitzvah. Consequently, if a non-Jew would want to fulfill the mitzvah of lulav or succah we wouldn’t stop him.
The Gemara in Berachos (51a) teaches that we don’t answer “amen” to the berachah of a Kuti unless we hear the entire berachah and know who he is saying it to. See Biur Halachah (215:2). Based on this, one would be allowed to help a Kuti make a berachah. He can’t make a berachah: אשר קדשנו במצוותיו וציונו על נטילת לולב – “Who sanctified us and made us holy with the mitzvah of taking the lulav”, as he was never commanded, but the berachah of shehechayanu he can make.
If this non-Jew would subsequently become a ger [convert] on Chol HaMo’ed Succos, he would now be obligated (for the first time as a proper obligation) to take arbah minim and sit in the succah and would have to recite the berachah of shehechayanu, just like a regular Jew who never made a shehechayanu on succah or arbah minim on Succos. So, we have a case where one would be obligated to say shehechayanu twice. (Chasukei Chemed, Succah pg. 213)
