You can claim multiple flights on the same day, they just have to have
different file names. What I suspect happened is that you erased the
logger after your first flight, and the logger started a new log file
with the same name. If you can, leave the old file in memory and the
logger will use a different name. If not, just rename the IGC file by
increasing the last digit in the file name by one. Do not change any
other characters that represent the date/mfg/serial code.

This also works if you made multiple X/C flights on the same log file
because you left the master on after landing. Again, claim the longest
flight as normal. Then copy the log file increasing the last digit by
one, and claim the additional flight by adjusting the start/end times
in the OLC web form. You will need to erase the waypoints and let OlC
re-optimize.