The error 'memcached cannot redeclare wp_cache_add()' is typically caused by a conflict between the Memcached object caching plugin and another object caching plugin, such as W3 Total Cache or WP Super Cache. This happens because both Memcached and the other caching plugin are trying to declare the same function, which leads to a redeclaration error.
To resolve this error, you can try one of the following solutions:
Disable the conflicting caching plugin - If you're using both Memcached and another caching plugin, try disabling the other caching plugin to see if that resolves the error. If you're not actively using the other caching plugin, you can safely remove it.
Use a different caching plugin - Instead of using Memcached, you can try using a different caching plugin that doesn't conflict with your other plugins. Redis Object Cache is a popular alternative that works well with WordPress.
Rename the conflicting function - If you're comfortable editing code, you can try renaming the conflicting function in either the Memcached plugin or the other caching plugin. This will prevent the redeclaration error from occurring. However, this solution may require some technical expertise and should only be attempted by experienced users.