Only one part file will replace the existing item and for other parts new IDs are getting generated.
There are some parts in Teamcenter and while importing the assembly these parts need to use "use existing" option.
The tcin_import utility replaces only one part with the existing item and the other parts are assigned new Item IDs.
SolutionImport will auto-assign a new ID if duplicate ID is found.
It is only allowed in case of revision is different and multiple revision is enabled, but it is not possible to create/update more than two new revisions at a time.
Importing multiple parts into the same item revision is not feasible.
Currently, the only way to achieve this is to import them in a separate import operations.