Ignore it unless there are other symptoms. You can activate the license a number a number of times before there will be any adverse issues (like errors saying it's been activated too many times). Tableau Tech Support can fix a license issue if it's been fulfilled too many times - it's a pretty typical issue. It is best practice to deactivate the license, but it isn't the end of the world if you don't.
We allow a small bit of wiggle room for just this issue - where it is not possible to perform a deactivation of the product key prior to using the software on a new machine. Some examples - IT wipes a machine or there is a crash which causes a re-install of the OS.
When moving the key to a new machine installation, best practice is to deactivate the key first. As mentioned above, not doing so will result in a maximum activation error and the key will not be usable.