I recently added a history course to my timetable, and I'm sure that it's the same time as a course I chose from before.. but MUGSI let me add it and confirm changes.. and save changes as well..
has this happened to anyone? should I change it or just wait..
Also, courses that have a term 3 description are full year with 6 units right?

Check your course timings on the Master Timetable.
Until SOLAR reopens for sectioning, it will let you add courses that conflict.
Best to change it now before things fill up
Term 3 is usually 6 units yes.

SOLAR isn't checking for conflicts right now, they're checking to see if you have all the required prereqs for whatever course you're choosing.
But yes, change it now because you won't be able to take that course

okay thanks so much guys

Check here to see if there are conflicts (only way to know about conflict before schedule release).