Yes, Redmi 6 can support 2 4G SIM cards at the same time. It is a dual 4G phone. It also has VoLTE support.
You can record up to 1080p resolution videos with Redmi 6.
Yes, Redmi 6 does have GPS support.
No. Redmi 6 does not come with the earphone in the box. You need to buy it separately.
If your budget absolutely can’t exceed the Rs. 8,000 mark, go with the Redmi 6 without any concerns whatsoever.