snap install with duplicated snap names gives confusing errors

[Paweł Stołowski]

If same snap name is passed to snap install by mistake, it gives confusing errors, e.g.

$ sudo snap install hello-world hello-world
error: cannot perform the following tasks:
- Download snap "hello-world" (29) from channel "stable" (rename /var/lib/snapd/snaps/hello-world_29.snap.partial /var/lib/snapd/snaps/hello-world_29.snap: no such file or directory)

It actually succeeds with the installation and just errors out on the 2nd snap.

Now, while having hello-world installed, doing it again with two same snap names produces:

$ sudo snap install hello-world hello-world hello-world
error: cannot install "hello-world", "hello-world", "hello-world": no install/refresh information
       results from the store

while it behaves ok with just one name:
$ sudo snap install hello-world
snap "hello-world" is already installed, see 'snap help refresh'

[Paweł Stołowski (stolowski)]

In the same vain, 'snap remove hello-world hello-world' removes the snap but gives funny error because of the duplication:

$ sudo snap remove hello-world hello-world
error: cannot perform the following tasks:
- Stop snap "hello-world" services (snap has no current revision)
- Remove security profile for snap "hello-world" (29) (cannot find installed snap "hello-world" at revision 29: missing file /snap/hello-world/29/meta/snap.yaml)
- Remove data for snap "hello-world" (29) (snap "hello-world" is not installed)

[John Lenton (chipaca)]

I think the right approach is:

* in snapstate (or in daemon, might be more straightforward there as it's before it's split into ops), all multi-ops should check for duplication and bail the whole thing early

* in cmd/snap, check for duplication and error nicely