Calculating an average over multiple entries

[AMSto]

Hello 

 The dummy of the data I have is shown below, I am trying to calculate the average project span which would be (20+10+20)/3. However, there are multiple entries for the same project, and when I try to use the normal average in the visuals, the number is incorrectly calculated. I have attempted to use the measure averagex(summarise... but this also does not work. 

In summary, how do I calculate the average over one column, when the value may be repeated multiple times in the column for different rows. 

I am still new to PowerBi so struggling to get to grips with everything. 

thanks