moby: service update "--mount-add" fails with "duplicate mount point"

Updating a service to replace a volume with a different volume fails with “duplicate mount point”

To reproduce:

$ docker service create --name=foo --mount type=volume,source=volume1,target=/foo nginx:alpine
foo

$docker service update --mount-add type=volume,source=volume2,target=/foo foo
foo

Then see tasks failing:

$ docker service ps foo

ID                         NAME             IMAGE         NODE          DESIRED STATE  CURRENT STATE            ERROR
0c7uelw2fk0qkeneww14g553m  foo.1      nginx:alpine  7e44720a5b2a  Ready          Rejected 1 second ago    "Duplicate mount point '/foo"
ckhrp1iq2rh5ehabqxw5992e2   \_ foo.1  nginx:alpine  7e44720a5b2a  Shutdown       Rejected 6 seconds ago   "Duplicate mount point '/foo"
38ycpjw10baw0lyfr7sjuhnul   \_ foo.1  nginx:alpine  7e44720a5b2a  Shutdown       Rejected 11 seconds ago  "Duplicate mount point '/foo"
1mz8x3nwhj3hrxtjlbtbic9y3   \_ foo.1  nginx:alpine  7e44720a5b2a  Shutdown       Rejected 16 seconds ago  "Duplicate mount point '/foo"
0k3oycgrzzurzoi8pvjg6teg0   \_ foo.1  nginx:alpine  7e44720a5b2a  Shutdown       Rejected 20 seconds ago  "Duplicate mount point '/foo"

$ docker inspect 0c7uelw2fk0qkeneww14g553m
...
"Status": {
    "Timestamp": "2016-08-16T21:09:40.972455571Z",
    "State": "rejected",
    "Message": "preparing",
    "Err": "Duplicate mount point '/foo'",
    "ContainerStatus": {}
},
...

Tested on current master, https://github.com/docker/docker/commit/641e1564cfeb3f94f68f3c43a353df10331afecf