Share notification may be sent to users who already have a share reference.

[raffis]

Describe the bug

A share notification may be sent to users who already have the share.

To Reproduce

1. Create share with three members (group or user)
2. Remove two members and add one new
3. the new gets a notification but also the first of the three.

Expected behavior

No share notification for the first.

Environment

• balloon server version: v2.6.1
• Deployed as: [e.g. docker,deb package,tar,compiled manually]
• PHP version: [e.g. 7.2.1]
• MongoDB version: [e.g. v3.4.18]

Additional context

Add any other context about the problem here.