How can you set the condition so that it can only occur one time in the pricing procedure?

If you have to allow a manual price and allow the manual price to be maintained.

For example: Condition type EDI1 - manual condition. You want to have the flexibility to be able to change
the condition value in the original entry, but not allow that same condition to be entered in the sales document twice.

First without configuring it, you can write a pricing requirement that could probably check for you.
 
or
 
In your configuration for the pricing condition, in the "changes which can be made" section, set
the "Manual entries" value to either "B" - automatic entry has priority, or "C" - manual entry
has priority.

or

Use Exclusion technique to avoid duplicating of conditions. Create Exclusion group and enter the
condition type with indicator as B which will take the best condition record i.e. the best discount
only will be applicable. Then assign this to Pricing Procedure.
The functionality is available in the following path:
IMG-->SD-->BASIC FUNCTIONS---> PRICING--->CONDITION EXCLUSION-->CONDITION EXCLUSION FOR GROUPS OF CONDITIONS
 

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