Multiple form submits on contribution pages

There have been 2 times (and possible more) where contribution pages have submitted more than once when a user filled them out. These have both been (on different pages) for a membership renewal using the "pay later" option. In one case there were about 6 contributions created, and 12 in the other.

This is what is showing in the apache log (edited to remove site info / ips, and removing css, etc):

[06/Mar/2019:14:37:19 +0000] "GET /civicrm/contribute/transact?reset=1&id=7&cs=114dc355fb504a48dddf1d51450c4a2d_1551882710_168&cid=605 HTTP/1.1" 200 23469 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/64.0.3282.140 Safari/537.36 Edge/18.17763" [06/Mar/2019:14:37:40 +0000] "GET /civicrm/ajax/l10n-js/en_US?r=0n8f3 HTTP/1.1" 200 2689 "https://mywebsite.com/civicrm/contribute/transact?reset=1&id=7&cs=114dc355fb504a48dddf1d51450c4a2d_1551882710_168&cid=605" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/64.0.3282.140 Safari/537.36 Edge/18.17763" [06/Mar/2019:14:39:15 +0000] "GET /civicrm/payment/form?formName=Main&currency=CAD&&is_back_office=&id=7&processor_id=0&cid=605&cs=114dc355fb504a48dddf1d51450c4a2d_1551882710_168&payment_instrument_id=undefined&snippet=json HTTP/1.1" 200 2046 "https://mywebsite.com/civicrm/contribute/transact?reset=1&id=7&cs=114dc355fb504a48dddf1d51450c4a2d_1551882710_168&cid=605" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/64.0.3282.140 Safari/537.36 Edge/18.17763" [06/Mar/2019:14:39:21 +0000] "POST /civicrm/contribute/transact HTTP/1.1" 302 619 "https://mywebsite.com/civicrm/contribute/transact?reset=1&id=7&cs=114dc355fb504a48dddf1d51450c4a2d_1551882710_168&cid=605" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/64.0.3282.140 Safari/537.36 Edge/18.17763" [06/Mar/2019:14:39:21 +0000] "POST /civicrm/contribute/transact HTTP/1.1" 302 619 "https://mywebsite.com/civicrm/contribute/transact?reset=1&id=7&cs=114dc355fb504a48dddf1d51450c4a2d_1551882710_168&cid=605" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/64.0.3282.140 Safari/537.36 Edge/18.17763" [06/Mar/2019:14:39:21 +0000] "POST /civicrm/contribute/transact HTTP/1.1" 302 619 "https://mywebsite.com/civicrm/contribute/transact?reset=1&id=7&cs=114dc355fb504a48dddf1d51450c4a2d_1551882710_168&cid=605" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/64.0.3282.140 Safari/537.36 Edge/18.17763" [06/Mar/2019:14:39:21 +0000] "POST /civicrm/contribute/transact HTTP/1.1" 302 619 "https://mywebsite.com/civicrm/contribute/transact?reset=1&id=7&cs=114dc355fb504a48dddf1d51450c4a2d_1551882710_168&cid=605" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/64.0.3282.140 Safari/537.36 Edge/18.17763" [06/Mar/2019:14:39:21 +0000] "POST /civicrm/contribute/transact HTTP/1.1" 302 619 "https://mywebsite.com/civicrm/contribute/transact?reset=1&id=7&cs=114dc355fb504a48dddf1d51450c4a2d_1551882710_168&cid=605" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/64.0.3282.140 Safari/537.36 Edge/18.17763" [06/Mar/2019:14:39:21 +0000] "POST /civicrm/contribute/transact HTTP/1.1" 500 900 "https://mywebsite.com/civicrm/contribute/transact?reset=1&id=7&cs=114dc355fb504a48dddf1d51450c4a2d_1551882710_168&cid=605" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/64.0.3282.140 Safari/537.36 Edge/18.17763"

The following is in the apache error log:

[Wed Mar 06 14:40:25.937577 2019] [core:error] End of script output before headers: index.php, referer: https://mywebsite.com/civicrm/contribute/transact?reset=1&id=7&cs=114dc355fb504a48dddf1d51450c4a2d_1551882710_168&cid=605

The site has the Stripe and CiviDiscount extensions enabled.

Any help is much appreciated!