If I am passing clear liquid at 8pm after first dose of suprep at 6pm, do I have to take second dose

Yes, you still have to take the second dose. I just had a colonoscopy and had the same experience as you -- running clear after first round so I wanted to skip the second round in the wee hours of the morning. I went ahead and did the second round because I was too afraid of not following instructions and having a bad result. Lo and behold, upon taking the second round of Suprep, more brown waste came out (not solid waste but definitely not at all clear until around 4 rounds of diarrhea that morning).

I was so glad I made the decision to go ahead and drink the second round, even though it was highly unpleasant. Maybe there was food still moving through my system that hadn't been in my colon yet during round 1. Anyway, my doctor complimented me on my prep. She found a very tiny polyp that she said she would have missed if not for my excellent prep. Bottom line: Drink the second round and ensure that you have a successful colonoscopy. It could save your life.