Duran's defense and speed on the bases ups his value, even against LHPs. He should play, everyday, at least until he proves he can't hit lefties well enough.
The problem is that Duvall is a pretty good fielder and had a longer track record of hitting LHPs very well. I'm not one to think a recent one month sample size after recovering from a wrist injury should weigh as heavily as some seem to think it should.
Kike is a very good defensive CF'er and has a career .804 OPS vs LHPs, but his lack of hitting since the end of 2021 carries more weight than Duvall's one month slump. I'm fine with Duran > Kike.
The real choice, to me, is not Duran v Duvall v LHPs, it's Duran v Casas v LHPs. (Turner to 1B, Yoshida to DH and Duvall in LF- not CF) The last time I suggested platooning Casas as a way to get other playing more, my head was nearly ripped off. There is good reason to keep Casas FT, too.
1. He needs playing time to further develop, especially on D.
2. He has hit LHPs (.735) slightly better than RHPs (.725), this year. Career is .695 v L.
Casas may be a slightly better hitter than Duran v LHPs, but it's pretty close, and Duran is a way better defender. Plus, platooning Casas improves our 1B D (Turner>Casas), LF D (Duvall> Yoshida) and CF D (Duran>Duvall.)
Another idea might be to spread the days off vs LHPs between Duran, Casas and Dugo. Also, make sure when Devers gets his scheduled day off, it's vs a lefty (Turner at 3B, Yoshi at DH and Duvall, Duran and Dugo in the OF.)
Like I said before, trading Duvall might end up solving the logjam with the short split side.