It's a tradition in my family going back generations to give the child two names and then call him/her by the second one, so as you say finding relatives has been quite difficult. In my case I find my first name so awful that it has blighted my life. I was named after a rich childless aunt, probably in the hope that she would leave me some money

. In the end she went a bit bonkers and crossed my brother out of her will and was on the point of crossing me out when she died. We shared what she left between us, something we had agreed between ourselves beforehand when we realised the way things were going!
My second forename has a shortened version which is used by all the family, but I wouldn't use that on a form, just the full second forename.