To avoid posting the inventory value twice (once with the item journal and again with the Post Inventory Cost to G/L batch job), you must set up your purchases and inventory adjustment as described here.
Open the General Posting Setup window.
Change the account number(s) in the Inventory Adjmt. Account field to the account to which the inventory valuation was posted.
Run the Post Inventory Cost to G/L batch job.
In the General Posting Setup window, change the account number(s) in the Inventory Adjmt. Account field to the account to which inventory adjustment will be posted in the future.
Registering Opening Item Entries
Post Inventory Cost to G/L Batch Job