Fix crash due to subscribing observer twice

[robertapplin]

Report to: Franz Demmel

Description of work

This PR fixes a consistent crash when using multiple of the Indirect and Inelastic interfaces. It is possible to cause this crash in multiple different ways, from multiple different interfaces. It is caused by adding the same observer to the notification center more than once, and the solution was easy - not to add the observer if it is already subscribed to the notification center.

Fixes #37071

To test:

1. Go to 'Inelastic'->Data Manipulations
2. Load data into the Symmetrise Tab
3. Click Preview, then click Run
4. Close the interface
5. Open the Data Analysis interface
6. On MSD Fit load data
7. Select Gaus function
8. Click Run. There should be no crash

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[jhaigh0]

replaced with #37074