I have installed the v4 tag on my forum. But it’s showing two global site tags.
I can’t guarantee that this is correct, but according to this post, the warning may be related to a bug in how GTM handles single page applications: [Solved] Multiple installations of Google Tag Manager detected. That post’s “What should you do” section suggests debugging the issue by checking your web inspector’s Network tab to see if more than one instance of
gtm.js is loaded.
The post also says that in the issue could happen if GTM preview mode is enabled - that is supposed to have been fixed though.
If you are unsure about checking how many instances of
gtm.js are being loaded, let us know and we’ll try to help.