Plasma 5.6.2, all current Arch packages (Antergos)
kdeconnect version is 0.9.g-1 
Android version is whatever is current on the play store, Marshmallow, LG G4

Two images:
Four text messages, three of which are double-notified:  http://i.imgur.com/2VoV82g.png

Two text messages, one of which is double notified, plus the "Messenger" notification from android triggered by one of them:  http://i.imgur.com/g8lW26s.png

Although this small sample size would seem to imply that emoticons are part of the equation I'm *fairly* sure I am seeing this with text-only SMS also.  The non-repeated SMS in the SECOND image above was sent from me to me as a test.

Essentially the problem is that most (not all) SMS notifications are displayed 2 times.  This wouldn't be so bad, but often the "Messaging" system notification is also displayed, and on lengthy messages this results in a wall of sometimes confusing text.

Reproducible: Sometimes

Steps to Reproduce:
1.  Receive an SMS
2.  Wait for the Notification
3.  ...
4.  Profit!

Actual Results:  
See pictures and description above, with double-notified SMS messages

Expected Results:  
Single notification of received SMS messages

I don't consider the Messenger notification to be a bug - seems to me that's a discrete notification of its own - though a way of filtering that out would be great.  However, the double SMS notifications are what I'm reporting here.

I have used kdeconnect on a few distros and DE's since it was almost brand new, and although I could be crazy I'm 90% sure I wasn't seeing this until my recent fresh install of 5.6 on Antegos.