Hi Atul,
As of my knowledge, there is no option to restrict the multiple entry of manual condition type in standard. An alternate way would be to maintain access sequence for the condition type and remove the manual flag for the condition type in the pricing procedure and maintain the condition type as "not possible to maintain manually" in M/06. The access sequence may contain the plant, for example. Maintain the condition record in MEK1 with blank value, so that the same will be proposed for the plant always with blank value. When the user want to enter any value, he can maintain only value, else keep it blank. Since manual entries are configured D (do not possible), then the user can not enter it multiple times in the PO.
If you still want it as manual and wanted to control the entry, you may go for custom routine. You may create the routine in VOFM transaction with the help of technical team and write the logic to check multiple entries and maintain the same in the "reqmts" field in the pricing procedure.
Refer the thread: http://scn.sap.com/thread/1279396 which explains the similar procedure in SD.
Regards,
AKPT