Inside the same FoxPro session (FPD, FPW or VFP) you can't open "low level" a file that is already open with USE.

USE xFile
FOPEN(xFile)

is the same as:

USE xFile
USE xFile in 0

To find out if a file is locked in another datasession inside the same VFP session, you can do:

SET DATASESSION TO ... <the other datasessionid>
?ISfLOCKED(<alias>)
SET DATASESSION TO ... <current datasessionid>