Currently the LastPass browser extension for Chrome/Edge does not allow Launching a second tab that includes the root domain if it is already open in another tab. The options become Fill and Copy UID or Password. The only way is to Edit the entry, manually copy the URL, open it in another tab, then use LastPass to fill the UID/PW.
Deciding to launch or not multiple tabs for a domain is not LastPass's place to decide. Launch and Fill should always be options, but enabled/disabled depending on the context. The Copy options should always include URL in addition of UID/PW.
In my work as a developer it is common that different sites or pages using the same root domain need to be accessed. Logmein.com is an example of that as well.