It's used in Attic also. ἵστημι is one of several verbs that have athematic forms in the perfect system, particularly in the plural. So, alongside the perfect ἑστήκαμεν, ἑστήκατε, and ἑστήκασιν, you also see ἕσταμεν, ἕστατε, ἑστᾶσι; and in the pluperfect, in addition to εἱστήκεμεν, εἱστήκετε, εἱστήκεσαν, there is ἕσταμεν, ἕστατε, ἕστασαν (even though some of these are identical to the perfect forms).
τέθνηκα and δέδοικα also have plurals in τέθναμεν and δέδιμεν, etc.
Dic mihi, Damoeta, 'cuium pecus' anne Latinum?