Aiii sabar je dulu mama, at 6 weeks memang sometimes susah nak tengok clearly especially kalau positioning baby awkward, a few of my mum friends went through the same worry jugak where doc cakap might be twins tapi belum confirm. Kalau memang kembar, usually by week 10-12 the second heartbeat and gestational sac akan jelas sangat, so just follow up dengan scan next week or two je and see if placenta atau baby yang kedua develop; the waiting part memang stressful but inshallah clarity akan datang soon! Best bet is stick dengan doc's plan and go for the next scan, they'll be sure by then