Duplicate requests for thumbnail

Description

After playing around with media Selector for a while, there are some duplicate requests.
When you click on a page, a few requests for thumbnail are sent. It might be leak, since
the requests are gone after reloading.

Step to reproduce:

Req:
1. Make sure the page "Alloy Plan" have a Teaser Image (e.g. Alloy Plan->AlloyPlan.png)
2. In Media pane, select "For All sites"(This step is to avoid interference from assets pane)
3. In All property mode, Reload the page.

Now we can reproduce the issue
4. In Content tab, click on Teaser image to open Media selector. See 3 requests to "http://sh-daily.ep.se/develop-alloymvc/EPiServer/CMS/Content/globalassets/en/alloy-plan/alloyplan.png,,105/Thumbnail?epieditmode=False" (The redundant requests here is not related to this issue.)
5. Click Cancel to close Media selector
6. Click on the Alloy Plan(Current Page) again on the page tree.

If you see extra requests to "http://sh-daily.ep.se/develop-alloymvc/EPiServer/CMS/Content/globalassets/en/alloy-plan/alloyplan.png,,105/Thumbnail?epieditmode=False", you are success to reproduce it. Now the requests will always be sent whenever you switch pages.
If you don't see extra requests, repeat step 4-6.

It is possible to reproduce the problem again and again, then more duplicate requests will be seen.