Take Two (1987)

On her deathbed a woman tells her son that 30 years before she had been a surrogate mother for a millionaire, and that he is that baby's identical twin. He goes to Los Angeles to try and get some of the inheritance, but his brother refuses to even talk to him. So he gets an apartment across from his brother and begins a stakeout as he writes a tabloid story about it, and begins to fall in love from a distance with his brother's wife.