Edit pipeline > No error checking on adding nodes

## Issue

You can add the same node multiple times. 

## Steps to reproduce

* Create template
* Create node
* Create pipeline, name it, and add the node
* Edit the pipeline > Click Add Node > chose the same node. (Repeat as many times as you like)
* Click **Save**
Result: the same node is added multiple times and displayed in the Edit Pipeline page, and the Pipeline grid



