We had made active serial number managment.  During production creation we are assigning a serial number to the production order.  System allows to assign the same serial number to the another production order also.

I have refered to the SAP note 647188 which gives solution like this:
When a serial number is assigned to a production order, a system status transaction (PMP1) is used - when the assignment is deleted, PMP2 is processed.  Thus, it is possible to define a user status (which forbids a second assignment) and set it for the assignment.

Can anybody tell me what is PMP1 and PMP2? and how to implement this. I am not able find any info regarding this.

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=== Though I cannot test and confirm, you can try the below procedure:

(1) Create a User Status Profile (or use an Existing one) with Equipment as the Object Type with 2 User Statuses: Order Assigned and Assignment Deleted

(2) For the User Status: Order Assigned
Select Forbidden (Influence) and Set (Next Action) as the Transaction Control for the Business
Transaction PP Order Assignment
Select Forbidden (Influence) and Set (Next Action) as the Transaction Control for the Business
Transaction PP Order Assignment

(3) For the User Status: Assignment Deleted
Select Forbidden (Influence) and Set (Next Action) as the Transaction Control for the Business
Transaction PP Order Assignment
Select No Influence (Influence) and Set (Next Action) as the Transaction Control for the Business
Transaction Cancel assignment to PP order

(4) Assign the above Status Profile to the Equipment Category being used by you.

Read Also:
Control Regular Entry for Material Consumption

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