The Common Sailor (Neptis hylas) looks very similar to the Common Sergeant (Athyma perius).
The major difference between the two is the white strip found on the leading edge of the forewing.
On the Common Sergeant that white band is a single long dash with three dots.
The Common Sailor, on the other hand, has a longer dash line and a single dot.

Chiang Mai area, Thailand, 25 December 2009