Here is what I found:
Below are the steps taken for the investigation:
a) Submitted a new app for approval.
b) App is approved.
c) Go to Edit>submit 3.0 App Release.
d) Submit same version of the app.
Actions performed by the system: message is shown that user cannot submit same app twice.
e) If we try to submit a lower version of the same app.
Actions performed by the system: It allows user to submit the lower version without giving any message.
Expected behavior: It should not allow user to submit a lower version of the same app when a higher version is already uploaded. It should show a proper message for the same.
f) If we submit a higher version of the app.
Actions performed by the system: It accepts the higher version without admin approval. A mail is sent to the user with a message 'Your app has been submitted! You'll be notified by email when your app has been approved.'
Clarification: Is it okay to submit a subsequent version of the same app without approval?
Expected behavior: The message which is sent to the user should be changed to 'Your app has been submitted! ' instead.

Regarding (e):

I think it should be OK for a developer to submit an older version, since different versions may be compatible with different versions of the platform (IGB). They may have some users who need to use an older version of IGB for some reason, and the developer may wish to support these users.

Regarding (f):

This seems like a bug, but we need to dig a little deeper.
Prutha - what user were you testing this as? Was that user a site admin?