(Pt. 1) The Butterfly Hill at Pulau Ubin, Singapore, was full of Glassy Tigers on 3 Aug 2019. Most were Blue Glassy Tigers (Ideopsis vulgaris macrina) [first pic] with a few Dark Glassy Tigers (Parantica agleoides agleoides) [second pic].
See Pt. 2 on how to distinguish between them.
(Pt. 2) A first glance, they look very similar. The main way to distinguish between them is to see whether the 'cell' on the forewing is 'broken' or not. If it is, it's a Blue Glassy Tiger. If not, it's a Dark Glassy Tiger. I capture one shot showing both butterflies feeding next to each other and circled the difference to make it clearer.