When one user is allocated to multiple projects/sprints at the same time (full time), then JIRA Portfolio does not give any indication of that to the administrator. It is possible to plan a user in multiple projects at the same time without getting any warning.
When you add a user to a team, Portfolio should indicate available hours based on the team members allocation to other teams. There should also be a way to clearly view the users allocation in all the other teams.
If a user is allocated for 40 hours in 2 teams, then there should be a warning message to make the user aware of this situation. To further add on, it would be best if we could have the visibility the total number of hours available for the team to ease on the scheduling and planning as well.