I recently bought apt 6 months ago while FTB. If I sell it I think I have no stamp duty clawback , but If however I buy a new apt and move into that as my new PPR, is there any stamp duty implication during the timeframe between trying to sell first apt and it not being my PPR?
Assuming you don't rent it out, no, though there may be CGT implications on any capital appreciation if you don't sell it within a year. If you do rent it out, you're liable for full clawback from the day your tenants move in.