Yur override file looks like a complete copy of the original unit file. This is completely wrong. override files are NOT meant to replace the original unit file, but instead are MERGED with the original.
If the original unit file has these lines:
and the override file has these lines:
Then the result used by systemd is:
And therfore, the error message is correct, as the result has 2 ExecStart lines.
Conclusion: Delete the override file:
sudo systemctl revert httpd.service
My guess is: The original reason for you to create the override is to fix this warning:
PIDFile= references a path below legacy directory
To do that, the override file should contain the ONLY the following 2 lines:
As a sidenote, the original does not use a pidfile, therefore this fix would not be required in the first place.
Perhaps, you better describe what you wanted to achieve by creating an override.
If you only want to add the dependency on php-fpm, then the override should contain (only) this:
And if you want to see the merged result of the original unit file and the overrride (sort of), then you can use:
sudo systemctl cat httpd.service
There is also:
sudo systemctl show httpd.service
but I find this rather coinfusing, because it shows everything (even defaults that are not explicitely set) and it does not show the config sections.