Originally posted by Latent Potential
Although they start of very differently in nature, it seems to me that... A)_French defence with a later ...,c5; leads to the same black pawn structure as... B) Sicilian with ...e6; and ...,d5;.
These are only similar if white chooses to play the 'closed" versions of the French or Sicilian, which are not that popular among better players. The main lines are not very similar, so I think the direct answer to your question is "Not too often, at least at the master level". But it does happen.
Open Sicilian 1. e4 c5 2. Nf3 e6 3 d4 cxd4 4 Nxd4
Main line French 1. e4 e6 2. d4 3 d5 3. Nc3 or Nd2
But if white plays 1. e4 c5 2. Nc3 e6 3. d3, you can also get very similar or identical positions to 1. e4 e6 2. d3 d5 3. Nc3.
As Robbie points out above, a lot of the 1. e4 c5 2. c3 e6 variations can also get positions that are similar or identical to positions out of the French. And if you are going to play c5 followed by e6, its best to know the French, too.