How to Remove Question Mark from between the strings

Somehow Google indexed one of the pages as
domain.com/string?string
How can I rewrite it to
domain.com/string
So, need to remove ?string from the url.
Tried many ways but didn't help.

Thank you in advance.

Already tried
RewriteRule ^string?string$ /string [L,R=301]
but it did not help.

You cannot match parts of the query string with a RewriteRule. That is clearly documented. And shown in an endless number of existing answers here on SO.

Instead you need the assistance of a RewriteCond and explicitly delete the query string:

RewriteEngine on
RewriteCond %{QUERY_STRING} ^string$
RewriteRule ^/?string/?$ /string [QSD,L,R=301]

In general you should implement such rules in the central http server's host configuration. If you do not have access to that (read: if you are using a cheap hosting provider) you can instead use a distributed configuration file (often named ".htaccess"), if the interpretation of such files in enabled in general and if you configured the option to overwrite rules in such files (see the AllowOverride directive in the documentation).

In addition you should consider why Google has been able to catalog such a URL. That means that the robot found a link to that URL somewhere. That is what you should take care of too. You still have to implement the redirection to prevent a scoring penalty, but certainly you want to fix the cause, not just handle the symptom.

This will remove any ?string from URL

RewriteEngine On
RewriteCond %{QUERY_STRING} ^(.+)$
RewriteRule ^(.*)$ $1? [R,L]