I totally agree with some of the previous comments! If I were adding Curb Appeal to this Duplex here is what I would do:
*I added a fun rendering mock up for visual explanation, things are not to scale so don't come after me! ;D
1. Definitely replace the 2 front doors with matching ones. I would even paint them a nice light teal to have them contrast with the existing exterior paint. The same goes for any missing windows or older doors. I would also consider either building out a deck like Unit 1 has or ditching it and making sure both front entrances have a small ramp for accessibility, in case some of your tenants are elderly or handicapped.
2. Landscaping- Add drought tolerate plants such as Lavender, Bougainvilleas, grass bushes, etc. It might be tricky to tell, but I added Bougainvilleas (in image) to help create separation between units. Also if you decide to add any type of front lawn, I suggest pavers and clover grass to help cut back on watering expenses. I also added window boxes for fun and to give you an idea. I am not too familiar with plant varieties and landscaping go to's for Colorado, so this might have to differ from what I suggest.
3. Definitely loose the swamp cooler/AC unit! You might want to consider a mini split or other unit on side of units as opposed to right in the front. Often these units are a killer for enhancing curb appeal!
4. Based off the photo, it appears the parking area is also in the front, which as part of 'Tenant Proofing' I would add some sort of railing or fencing to prevent tenants from parking too close to the property. This will also help keep any children safe and create more security for tenants and you as the property owner.
5. Lastly are some additional details. I would add an architectural medallion to the top front since it looks a little bare. I would also swap old lighting fixtures for new ones, I chose copper ones to match the roof. If you have the budget, definitely add a new layer of asphalt and create designated parking.