The apologism/dismissiveness on this thread is frankly embarrassing.
What exactly do people think racism is? It isn't always bellowing explicit slurs at people. It also takes the form of microaggressions that may or may not be intentional (everyone surely knows about unconscious bias these days) but it doesn't mean OP should have to deal with it. It's for her colleagues to sort themselves out!
In this case, to spell it out, the micro aggression is people (not of the same ethnicity as OP) not valuing her identity or individuality enough to bother learning and remembering her name. This is because on some level they see her (and her other South Asian colleague) as being "lesser than".
It may be subconscious. But put it this way, they aren't mixing up OP with a white colleague she sits next to are they? They're confusing her with the only other South Asian in the office.
It shows that they're reducing both of these women to their ethnicity. When colleagues see OP that's the predominant thing they see...not her role, or what team she's part of, otherwise they'd be able to distinguish between the two South Asian colleagues! And that isn't just "lazy" - it's rude and it's a form of prejudice.
Now - in terms of what to do? I liked PP suggestion "but we look nothing alike". You could also just look very blank and say any of the following:
"who are you talking to?"
"no, she's the other one" (and point to your colleague)
"that's not my name"
"why do you confuse me and X?"
And let them squirm. Let the silence grow uncomfoetabke. **It's not your job to bail anyone out.