Normally we say “sofek d’rabbanan l’kulo” and will be lenient in a case of a doubt that is only an issur d’rabbanan. I have heard that there are poskim who say this doesn’t apply in the case when there is a shailo of a “sofek d’rabbanan” by the tevillah of an isha that is a Niddah (for example a shailo of a chatzitzah) since this is a “davar sheyesh lo matirim”. She would be able to easily go back to the mikveh and tovel again. Therefore we don’t say “sofek d’rabbanan l’kulo”. Who, is anyone, says this? Any sources? Any poskim that apply this l’maseh (perhaps found in Teshuva seforim or other halacha seforim.)
Answer:
The Gemara (Beitzah 3b-4a) writes that for a “davar she-yesh lo matirin” we do not follow the principle of safek de-rabanan le-kula.
The principle is ruled by the Shulchan Aruch (Orach Chaim 513:2 and Yoreh De’ah 102:1).
Tosafos (Eruvin 45b) mentions that for some matters the Rabbis were nonetheless lenient.
Poskim discuss whether this principle applies even for a rabbinic mitzvah, or whether it is limited to a rabbinic prohibition.
Best wishes.
