I also share a first initial with my sister L. It is annoying.
We found it funny when we were small, then annoying as teenagers, there was a stage when we would open each others post to wind each other up!! Now its not a problem as we no longer share an address.
My mum tried to solve this issue by giving us middle names,(it didn't work) suddenly having a middle name a 6 years old means that some places have my midddle name, and some don't. I was aloud to choose my middle name - I now hate it!! and as is not official I no longer use it.