Comment 1 for bug 1593435

I'm seeing a similar, I believe, issue.

$ snap --version
snap 2.25
snapd 2.25
series 16
linuxmint 18.1
kernel 4.4.0-75-generic

I have the core_2312.snap already downloaded and stored on /var/lib/snapd/snaps/ (an NFS mount shared amongst multiple computers).

$ snap info core
name: core
summary: "snapd runtime environment"
publisher: canonical
contact: <email address hidden>
description: |
  The core runtime environment for snapd
type: core
channels:
  latest/stable: 16-2 (2312) 83MB -
  latest/candidate: 16-2.26.8 (2329) 84MB -
  latest/beta: 16-2.26.8 (2329) 84MB -
  latest/edge: 16-2.26.8+git261.f737a92 (2349) 85MB -

File already downloaded
$ sha256sum core_2312.snap
d29538b2294b6f681294e264d5c3b0f9668b1ff16eb9607c1bcd8b89e86a3695 core_2312.snap

Just downloaded from https://public.apps.ubuntu.com/anon/download-snap/99T7MUlRhtI3U0QFgl5mXXESAiSwt776_2312.snap (Snap store, latest "stable")
$ sha256sum 99T7MUlRhtI3U0QFgl5mXXESAiSwt776_2312.snap
d29538b2294b6f681294e264d5c3b0f9668b1ff16eb9607c1bcd8b89e86a3695 99T7MUlRhtI3U0QFgl5mXXESAiSwt776_2312.snap

When I try `sudo snap install core`, snap tries to re-download the file, even though I already have it. I have a very limited Internet connection, so I download files elsewhere and bring them home to share among my various computers.