GRE tunnel key
When you use the same source interface for multiple GRE tunnels, your router won’t know what tunnel to use to de-encapsulate packets. This issue can be solved by implementing what is known as a tunnel key.
A tunnel key adds a 32-bit unique key ID to the GRE header. By configuring a unique tunnel key on each tunnel interface, routers are able to distinguish between such GRE tunnels.
When a router receives such a packet, it de-encapsulates the packet first, then it does a match on the destination IP address, and then it matches the tunnel key.
For more information on the particular behavior of this feature, take a look at GRE tunnel key operation with only one tunnel key configured.