Look OP in laymen's terms, effectively there are 3 different cases of "duplicated numbers" in the Interstate system.
1) three digit interstates
3DIs can be duplicated out of state. However there are several cases where these rules are skirted and there are "de facto" duplications in-state for a few hundred feet because of technicalities such as spurs multiplexing with parents and other oddities.
2) suffixed interstates
There are a few examples of suffixed interstates where an interstate either splits its mainline, or spurs an important connection to another road or city. These have not been officially sanctioned in thirty years, though new ones are occasionally still created. Currently I-35 has two mainline splits in Texas and Minnesota that both rejoin. During these splits, labeled 35E and 35W, the mainline I-35 goes away. Also in Texas, I-69 is under construction, but the setup will be similar, with a 69E and a 69W branching off. However in this example, the mainline will at first disappear, but then reappear and split the middle of 69E and 69W, with this section labeled C for central. And in Ohio, I-480 has a major two-mile freeway branch to the northeast, called 480N, which isn't shown on guide signs but appears on the road's reassurance/info panels.
3) "split" interstates
These are regional interstates that are mainline (2 digit) interstates, but are far enough away from another regional interstate on the other side of the grid that both can share a number. A great majority of these cases are east-west, with one road in the west or Midwest and one in the east, with one future example of a north-south split. The official ones currently are I-76, I-84, I-86, and I-88, all east-west. I-74 is a "de facto" example, where there will never be a connection between a complete western section, and an under construction, but close to complete, eastern section hundreds of miles away. The intermediary roads will never be built. I-69 may end up suffixed and split if construction of its middle sections stalls. Future examples may be I-66, which would be an east-west split, and I-87, which would be the first north-south split.