Carbon offset programs apply a number of methods to ensure that offset credits convey an exclusive claim to GHG reductions.
Double issuance is avoided primarily by:
- Ensuring that offset credits are only issued after program approval of emission reduction verification reports and other supporting documentation;
- Checking that the accounting boundaries used to quantify GHG reductions for different projects do not overlap
- Actively monitoring project registrations – including at other programs—to check that a project is not issued credits by more than one program for the same emission reductions.
Double use is avoided primarily through registry systems that assign unique serial numbers to individual offset credits, track their transfer and ownership, and record the purpose of their use and retirement.
Double claiming is avoided though:
- Restricting the eligibility of project types (e.g., excluding those are known to be subject to GHG reduction mandates or competing claims); and/or
- Requiring project developers to sign legal attestations asserting exclusive claims to any credited emission reductions, and agreeing to legally convey such claims to the buyers of offset credits.
Offset programs are still deliberating on how to reconcile competing claims for GHG reductions that are covered by countries’ climate action pledges under the Paris Agreement (see Carbon Offsets after 2020: the world under Paris for more info).
 Procedures may include requiring project developers to sign legal attestations stipulating that they will not request issuance of offset credits for emission reductions from more than one program (unless they are effectively “transferring” credits from one program to another).
 Some third-party programs, like Green-e Climate, provide checks on credit retirement steps for retail credit buyers. However, in most cases this adds little value beyond what carbon offset programs already make available to any buyer in terms of retirement certification.