Two Sn2 processes mean retention. A classic example of this is found in the synthesis of alpha-carboxylic acids from amino acids. One diazotizes the amino acid, typically with sodium nitrite in water. This species is quickly attacked by the carboxylate group of the diazonium derivative, expelling nitrogen and resulting in the formation of a three-membered alpha lactone. Water is easily a good enough nucleophile to open this highly strained ring by a second Sn2 process, resulting in retention of stereochemistry at the alpha-carbon (say, 20:1 dr for the reaction run in water with a sulfuric acid catalyst). This is all one pot, of course... and moreover, the intermediates are not isolable.