I've noticed contracts in which Attachment numbers and Exhibit numbers are repeated on the same contract. For example, on one contract, there could be Attachment# 0006 on the base contract designated as a CDRL and Attachment# 0006 on a contract modification designated as a GFP Attachment. DFARS PGI 204.7102(b) Numbering exhibits and attachments states "(2) Once an identifier has been assigned to an exhibit, do not use it on another exhibit in the same contract." Based on this excerpt, is only one Exhibit and Attachment "number registry" allowed per contract in section J, to include both the base contract and contract modifications, with no exhibit or attachment numbers being repeated?
When I checked Acquisition.gov, I could not find DFARS PGI 204.7102, but did find DFARS PGI 204.7105 does have guidance for contract exhibits and attachments. (b)(2) states "Once an identifier has been assigned to an exhibit, do not use it on another exhibit in the same contract." It does not make the same statement regarding attachments. All I could find about attachments was in (5) which simply states "use numbers to identify attachments" and (a)(7) which states "the contracting officer may append attachments to exhibits, as long as the attachment does not identify a deliverable requirement that has not been established by a contract line item or subline item or exhibit line item." It's also important to note that FAR 4.1006 (a) says, "when a new item (such as an increased quantity) is added to the procurement instrument (by way of a contract modification), assign a new line item number."
To answer your question, I agree the DFARS makes it clear that you do NOT use the same identifier that was used on one exhibit for another exhibit on the same contract. However, the DFARS does not make the same declarative statement about attachments. I think the reason why is due to the need to have a little more flexibility with attachment numbering. One example of when flexibility might be needed is attachments appended to exhibits. When more than one exhibit in the contract has attachments appended to it, an attachment to one exhibit could have the same number as another entirely different attachment on a second exhibit. This is okay because it's the exhibit that is tied to a deliverable and attachments normally are not. Another situation where some flexibility might be needed is a modification that establishes a new line item on the contract. It's possible that one or more attachments need to be added in support of the new line item in the modification. One or more of these new attachments could technically have the same number as one or more attachments already in the contract. While using the same number for an attachment in a modification as an attachment already in the contract is not what I would prefer to do, it's not prohibited by the FAR/DFARS. Another example is indefinite delivery contracts. Depending on how the contract is structured, a delivery order or task order issued under the contract could have one or more standalone attachments that only apply to that specific order. Therefore, it's possible that the same number used for an attachment on one deliver or task order is used for an attachment on another order. These are just a few examples, and there could be others.
Bottom line: the DFARS guidance is much more restrictive for numbering exhibits than it is for numbering attachments. As just pointed out, there are some situations where the same number can be used on more than one contract attachment. However, each contract exhibit needs to have it's own exhibit number which follows the criteria for numbering exhibit line items in DFARS PGI 204.7105(c).