Microsoft lifted the block on assigning two Exchange Online licences to the same user. The error goes away, the mailbox keeps the best plan — and every licence assigned is still billed.
Microsoft changed how Exchange Online licensing works in a way that quietly removes a long-standing operational irritation — and quietly increases the risk of paying twice for the same mailbox.
Tenant administrators can now assign multiple Exchange Online licences to the same Microsoft Entra ID user. This brings Exchange Online into line with SharePoint and Teams, which have supported concurrent licence assignment for some time.
Previously, assigning more than one licence pack containing an Exchange Online licence to the same user threw an exception. It did not matter how you tried it — PowerShell, group-based licensing in Entra ID, or the Microsoft 365 admin centre — the message was the same and the change was rejected:
“You can’t assign licenses that contain these conflicting services; Exchange Online (Plan 1), Exchange Online (Plan 2). Review the services included with each license and try again.”
That error appeared for any combination of the following assigned to the same user:
Under the new behaviour, the combinations Microsoft publishes as stackable can be assigned concurrently.
Legacy products that are no longer sold but may still be in production are outside this change — Microsoft 365 Small Business and Mid-sized Business among them. It also does not apply to licence packs bundled through British Telecom.
Exchange Online works out which of the actively assigned plans is the most senior and applies that plan’s features to the mailbox. Features here means mailbox quotas, transport limits, protocol access and similar.
When a licence is removed from the user or expires, the remaining active licences are re-evaluated, a new senior plan is established, and its features are applied. Demotions and promotions are both supported, and Exchange Online maps the licence pack names shown in Entra ID and the Microsoft 365 admin centre to capabilities.
The order of seniority, from least to most, is:
Before the change, this had to be managed by hand. Administrators raised incidents with Microsoft and manually removed the older Exchange Online licence before the new pack could be assigned. Where the sequence was not followed exactly, the Exchange Online mailbox could end up soft-deleted — a support ticket generated by a licensing rule, which is nobody’s idea of a good afternoon.
Here is the part to take seriously. The technical conflict is gone. The commercial one is not.
Every licence assigned is billed. The mailbox will happily run on the most senior plan while you pay for the junior one sitting underneath it. Previously Microsoft’s own error message stopped that from happening. Now nothing does — the platform will let you stack licences indefinitely and invoice you for all of them.
So the reporting burden has gone up, not down. You need to know which licences are assigned to which users, which of those assignments are redundant, and which are simply unused.
That is what CerteroX SaaS Management is for. Its Microsoft 365 connector pulls the authoritative user and licence list directly from the vendor rather than inferring it, so the assignment picture is the tenant’s own record. Unused licence detection flags seats with 30 or more days of zero usage. Cost per licensed user is reported alongside cost per active user, which is where stacked assignments show up as a gap between the two numbers. Reclaim is available as a bulk action and as a workflow action, so the remediation does not have to be manual either.
Microsoft removed a guardrail. Replace it with a report you actually read.
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