Interdisciplinary studies is usually a major composed of work in more than one department. Definition depends on the school. In some cases, it may include student-designed majors with approval of the faculty.
Double major means having two majors.
Dual degree may refer to one of several things:
Double major (sometimes specifically in different divisions for schools divided into divisions like arts & sciences, engineering, business, agriculture, etc.).
Two bachelor’s degrees at different schools in a combined plan, such as 3+2 LAC+engineering programs.
Combined plan with a bachelor’s degree and a master’s degree or professional degree like an MD.
For an interdisciplinary studies major/degree, you will probably have about the same workload as a regular single major - about 36-45 major credits. It's just that those courses will take place across two or more departments and be integrated into one program.
For a double major, you will have more coursework - perhaps twice as much major coursework depending on how related your courses are. For two closely related majors (like biology and chemistry) you might have anywhere from 51-66+ credits to take; for two unrelated majors (like biology and Spanish) that's 72-90 credits we're talking about (give or take). With careful planning you might be able to finish a double major in 4 years, but sometimes it takes longer.
Generally speaking I have heard dual-degree plans as being two degrees from two different schools. These programs almost always take at least 5 years to finish.