Is it permissible for someone performing 'Umrah to go to Jeddah after 'Umrah, then return and enter into a state of Ihram for a second 'Umrah? And what is the Miqat for the people of Jeddah?

It is permissible to perform more than one 'Umrah in a single day. It is valid to enter into the state of Ihram for 'Umrah from Jeddah because it is within the Miqat. However, for someone who wishes to perform 'Umrah while already in Makkah, they must enter into Ihram from the nearest point outside the Haram, such as Tan'im. If one enters into Ihram from within Makkah, the 'Umrah is valid, but a sacrifice (dam) is required.