The dmd source code in template.c:
-------------------------------------------------------
void TemplateDeclaration::semantic(Scope *sc)
{
    [...]
    if (Module *m = sc->module) // should use getModule() instead?
    {
        // Generate these functions as they may be used
        // when template is instantiated in other modules
        // even if assertions or bounds checking are disabled in this module
        m->toModuleArray();
        m->toModuleAssert();     <=============
        m->toModuleUnittest();
    }
---------------------------------------------------

This results in huge numbers of assert() functions to be generated, each with its own copy of the source file name string. The assert() function is generated as a COMDAT, which removes duplicates, but the filename strings are not put in COMDATs and are duplicated innumerable times.