shouldn’t ἐάν be always followed by subj.? In Stephanus, however, it is followed by indicative: ἐὰν γὰρ ἐστι τὸ αὐτὸ μὲν ὑποκεῖμενον, οὐκ ἔστι δὲ τὸ αὐτὸ κατηγορούμενον, οὐκετι γίνεται ἀντίφασις.
If the subject is the same but the predicate is not the same, there is no more contradiction.