I'm getting the double-entry bug almost all the time with Firefox 74 on Fedora 31 x86-74. Hitting R, reloading the page, or re-starting firefox don't fix it. About 1/3 of the time the first box is OK. I don't think I've seen the second box OK yet. [Edit} Just had one where the first 3 boxes were OK. (I was too slow on the third but entered it posthumously. The 4th had double chars.)
OK OK. I was clicking in each box. (Didn't read the instructions) This last time I swear I entered the correct value for the [4th or 5th] pit before the jump but still fell in. I'll report if I can duplicate.