What insect is this? I saw it in my house in Kolkata on 2 April 2018, at 8:00 p.m
I would concur with user22542 --> order Diptera: family Dolichopodidae

Supporting evidence outside of wikipedia is on an Australian website. http://www.bowerbird.org.au/observations/35021

I don't suggest them to be the same species, however potentially related as they seem to live in grassy places across the globe.