The filename isn't the fully specified filename. You can have many files with the same name under different directories. The filename consist of 'drive:\path...\file'.

ex.
===
SQL> create tablespace users2 datafile 'C:\ORACLE\ORADATA\MAGDB\DATA\USERS201.dbf' size 1M;

Tablespace created.

SQL> create tablespace users3 datafile 'C:\ORACLE\ORADATA\MAGDB\DATA\USERS201.dbf' reuse;

ERROR at line 1:
ORA-01537: cannot add data file 'C:\ORACLE\ORADATA\MAGDB\DATA\USERS201.dbf' - file already part of d

BUT if you copy the file 'C:\ORACLE\ORADATA\MAGDB\DATA\USERS201.dbf' to
'C:\ORACLE\' then

SQL> create tablespace users3 datafile 'C:\ORACLE\USERS201.dbf' reuse;

Tablespace created.

Works fine!