You are likely referring to the "de-duping" of multiple inquiries, which is a function of the FICO scoring algorithm, and not a process by which an inquiree controls how many inquiries are made or recorded in your credit report.
The FCRA requires that a record be kept of each inquiry made to a consumer's credit file. It is a legal requirement based on their obligation to provide a record of prior inquiries made within certain periods of time. An inquiree can agree to only make one inquiry, but cannot agree to prevent any permissible inquiry from being recorded in a consumer's credit file. The only control by an inquiree is the possible coding of multiple inquiries so that only one is "hard," thus excluding the additional inquiries from being shown in credit reports available to others.
As a separate credit scoring matter, the FICO scoring algorithm has a process they refer to as "de-duping," wherein the algorithm only considers, for scoring purposes, multiple auto, mortgage, or student loan related inquiries made within a sliding window (usually a 14 day window) as only one inquiry for scoring purposes. However, each inquiry appears in the consumer's credit report.
For the same reason that FICO only scores multiple inquiries as one, most potential creditors will, upon seeing multiple auto, mortgage, or student loan related inquiries, not consider that as a subtantive factor in their lending determinations.
