Yuna look-alike was the generalized reputation given to Lenne when FFX-2 was first being promoted and not much was known about the story. Yuna and Lenne don't look alike. The only reason some people are deluded into believing they are similar is because of a certain promotional image circulated online which showed Lenne and Yuna in songstress outfits side by side. When placed side by side in similar costumes and backgrounds, the illusion is created that the two resemble eachother. Give Lenne credit for the great character she is, not as a "Yuna clone". Same goes for Shuin, a complicated and fierce personality, not just "that Tidus lookalike".
The connection between Yuna and Lenne isn’t made all too clear in the beginning of the game, but it’s quite obvious that they share one. Lenne and Yuna share a body through the Singer Sphere due to the fact that Lenne’s spirit or essence is within the Singer Sphere. Due to the fact that the Singer Sphere holds the essence of its previous owner within it, Lenne is able to be connected Yuna and even transform Yuna into looking like her at times… this also probably the reason why LeBlanc was able to transform into Yuna in the Real Emotions FMV.
The connection between these two is so strong that Lenne’s emotions can even influence Yuna’s. Lenne’s memories also at times assault Yuna’s dreams, causing Yuna to place herself in Lenne’s position within the dreams.
In terms of whether or not Lenne is a “Yuna Clone”… well I kinda find this idea rather ridiculous. Sure, they have a lot in common, in fact, Lenne was probably designed to parallel Yuna. Lenne and Shuin’s story is one that ended in tragedy, but eventually they finally had their happy ending. This is meant to be a lesson for Yuna to learn about mistakes of the past and preventing them as well as learning how to be strong despite the loss of a loved one.
Asides from the hair, Yuna really doesn’t bear too much of a resemblance to Lenne. Though supposedly Lenne is nineteen, I really believe that she looks much older. Lenne’s hair, while nearly the colour as Yuna’s, is much longer and without bangs. Lenne’s features are more defined than Yuna’s, giving her an older, mature look while Yuna still looks very young and despite her gunner outfit, naïve.
The similarities really stop there, there are a few more bits and pieces here and there, but what people don’t share similar traits with others. While Shuin may have a deeper connection to Tidus, Yuna just really serves as a vessel for Lenne until past problems are solved.