Err. during GRN

Hi,

I'm getting err. message during making GRN against PO no.

Err. are

1. batch xxxxx has already been posted in stock

2.batch xxxxx has already been posted in stock

3.Shortfall of 56 days against SLED in current item (20.05.2008)

Kdly any guide me to over come this.

thks & regards,

G