When two or more duplicated curves/sheets/solid are active and on the top of each other in the same exact position, it would be great to have a warning message or to hightlight those bodies or curves in the viewport/outliner while selecting them. This would avoid failure when using a tool/command and not knowing what's going on.

In Siemens NX for example, there's an Analysis/Assembly Clearance command to perform the analysis that gives a hint of where possible overlappings can be.

A tool that virtually cuts the model to check overlapping geometries. See Shap3r, Spaceclaim and many other cad programs.

A similar drop down menu like in Fusion when you hold left click on faces to let you choose which face you want to select would be cool.