Will be travelling back Bangkok , Dubai to UK.
I buy Duty Free whisky at Bangkok Airport and they scan my boarding card. Then if i buy whisky at Dubai Airport, and hand over my boarding card will they know I have already bought whisky at Bangkok Airport and refuse me?
You can buy "as much duty free as you want" and from where ever you want. The only limits are imposed when you arrive back in the UK (assuming you are only transferring in Dubai. If stopping over outside of the airport, they may have their own rules)
------------- It'll work out in the end!!!!
I didn't do it !! Nobody saw me do it !! You can't prove anything !!
Make sure you tell them at Bangkok duty free that you are landing at Dubai as what you buy has to go though the security check with your other carry on bag before you board for the second leg at Dubai. The bottle bag has to be sealed with the receipt inside it or they'll confiscate it (more than 100 ml). This happened to me - Heathrow didn't seal my bag properly, LA took it out of my cabin bag & away.
The duty free limit from outside EU is one litre, doesn't matter where you bought it. So if you get another bottle at Dubai, you declare it or brazen it out through the Green channel with 2 duty free bags hidden in your carry-on bag: your choice. You might be better buying supermarket booze in Bangkok, if it's cheaper than UK. One litre weighs a kilo, so you'll know how many bottles you can pack into your hold luggage.
Good idea checking prices. UK supermarkets always have offers on malt whisky and I have never found a saving large enough to warrant buying it in airports.