From Wiktionary, the free dictionary
σῠρ- • (sur-) occurs before ρ (r)
- Alternative form of σῠν- (sun-)
συρ- • (syr-) or σύρ-
- Alternative form of συν- (syn-) occurs before before ρ (r)
- συν- (“together”) + ρέω (“flow”) → συρρέω (syrréo, “flock”) /siˈre.o/
- the two ρρ are pronounced as one