In PostgreSQL, the same function name can be used for different function definitions if the functions are placed in different schemas, or if the number of arguments or their types differ. Tcl, however, requires all procedure names to be distinct. PL/Tcl deals with this by including the argument type names in the internal Tcl procedure name, and then appending the function's object ID (OID) to the internal Tcl procedure name if necessary to make it different from the names of all previously-loaded functions in the same Tcl interpreter. Thus, PostgreSQL functions with the same name and different argument types will be different Tcl procedures, too. This is not normally a concern for a PL/Tcl programmer, but it might be visible when debugging.
For this reason among others, a PL/Tcl function cannot call another one directly (that is, within Tcl). If you need to do that, you must go through SQL, using spi_exec or a related command.
Read the upstream documentation. For corrections, first check the current version.