I had the same problem on a ASA 5516-X with a multiple context setup.
I wanted to configure a dhcprelay on Context-A, forwarding the DHCP requests through Context-B to a DHCP server.
(Client Network <Context A> Transit Network <Context B> DHCP server network)
The interfaces of both contexts in the transit network were configured at the same physical port as a VLAN subinterface. Regular network communication between these interfaces was possible.
When trying to configure the "dhcprelay server 10.0.0.1 inside" at Context A, I got the same error as you:
This interface is shared and cannot be configured with DHCP.
So I deleted the interface at the context B, configured successfully the dhcprelay at context A and tried to configure the interface again on context B. But if i tried to configure the nameif, I got a similar message at context B:
This interface will not be shared an DHCP has already been configured on this interface in another context.
Please clear all DHCP configuration on this other interface and try again.
So this did not work.
I did not find a real solution, but at least a workaround:
I used another network link to the ASA and presented the Transit VLAN as the untagged/native VLAN to this interface.
Context A uses the initial VLAN subinterface, Context B uses the additional interface without VLAN tagging.
Definitively not a nice and proper solution, but at least a workaround... Probably it helps somebody else.
