In case a petition has been filed with the clerk pursuant to section 8, and subsequently another petition is filed by other petitioners proposing to affect the same territory in whole or part, then the subsequently filed petition shall not be submitted to the electors while in conflict with the prior petition: Provided, however, That if such prior petition on file is one on which the board of supervisors has not finally set the date for an election, and such subsequent petition has been filed as a substitute therefor encompassing all of the same territory and having among its signers at least 4/5 of the qualified petitioners shown on such prior petition, then the board shall act on such subsequent petition in the place and stead of said prior one. If the board finds that said substitute petition complies with the provisions of this act, an election shall be called thereon; otherwise the election shall be held on such prior petition if it complies with this act.