Davey126 is broadly right. Syncdocs does not know that other users of Syncdocs have a copy open on their local PC.
hat happens when multiple users working on the same file synchronize this file with Google Docs ?
Depends on the type of file on Google Docs:
1. The file is not in Google Docs editable format, say a jpeg. Then the multiple versions of the document are uploaded. If versioning is enabled (the default) then the same versions are all in the file history. If you right click on the file in Google Docs, you can see who uploaded which version, and when, but the changes are not merged into a single master document.
2. The file is in Google Doc editable format. Google will merge changes, but normally not very well. The version history is selectable in the Google editor, and you can manually merge the changes.
3. The file is in MS Word or a Google document and the local users are editing in MS Word. Then changes are merged using Word's merge facility, which is good, and you can accept or reject changes from various users.
As Davey says, it can get complicated. Google Docs solution is collaborative editing, where both users view and edit the same document in their web browsers.
Is it possible to block the opening of a shared file to a user when another user has already opened it locally via syndocs ?
No. If the file was shared so that both users can access it, then they can both edit it, and the changes are synced back as described above. It is an interesting idea, though.