Originally Saint Paul’s church, dating to 1430, was the main cathedral in this neighborhood. But it was destroyed in the Revolution.
So Napoleon eventually designated this one, then called Saint Louis and dating from 1627, to be the new center of worship for this corner of Paris.
But as a compromise, the local bishop eventually convinced him to add Saint Paul to the name, in memory of the one old one. Pleasing the parish, even if the final name is a mouthful.