I have seen people reacting with horror if they see someone else cosplaying the same character as themselves. Everyone is entitled to feel and react as the want, but here are a few of my thoughts on the topic.
I actually love meeting people who cosplay the same as me, sure you compare a bit, but you also get ideas and inspiration to make your own better, but best of all, you get to talk to other cosplayers who love the character as much as you do.
Three Isabela makes for more fun. Photo by Katrix
What about competitions!
Well, my mantra is that cosplay is fun and I also think competitions are fun. Most people entering competitions want to do their best and try to win (myself included). And sometimes you end up in a mix with a character double ganger.
Yes, it is a competition, but if you win or not should not be based on you having the best version of "X" character, it should be that you have the best craftsmanship or stage play out of all competitors no matter which costume they wear. With this in mind you can push yourself even further (if winning or great craftsmanship is your goal).
For DreamHack Summer 2015 I knew that Ciwiel wanted to make a Mercy as well. My first reaction was that I should share what I knew with her and in return ask her questions of how she planned to solve other "problems" that I wanted input on. Also when I found the reference kit from Blizzard, I shared it with her right away (what kind of cosplay friend would I be if I hid source material).
In the end she won second place at DreamHack. She had included details that really gave a great profile on stage, really giving a presence of her cosplay and her wings were so gorgeous. And I felt a little proud, knowing I had had a little influence on her motivation to finish and in the solutions she chose. For my own part I got new ideas on how to upgrade my own.
Ciwiel on stage after winning second place. Photo: DreamHack