I'll suggest asking yourself a few questions.
[I'm using 'it' to refer to the character to underline that characters are things we create rather than real people or their alter-egos. Unless you want it to be an alter-ego, obviously.]
Q1. If the character turns out to resemble you physically, stop and give it a moment of thought. Why did you make it your twin?
A1.1 Because I want it to look like me. Wrong: change hair and eye colour, and height or weight.
A1.2 Because I like characters that have that specific physical appearance. OK: try to change at least hair style or hair colour or height. Try to pass it off as a cousin rather than a twin of yourself.
A1.3 No reason. It just happened that way. OK: decide if the story really needs that particular appearance and then change as much as you feel comfortable. Or go overboard and try to make it as different from you as possible.
Q2. Does it have the same interests as you? The same hobbies? The same political/sports/literature/musical/etc likes?
A2.1 Don't bother to ask yourself why. In the best case scenario, you want to 'preach' something you believe in to your readers. If that's the case choose one or two things you want to preach about and which correspond to your personal views. Get rid of the rest. Either don't mention those interests at all, or give it different tastes (not necessarily opposing tastes: if you don't like jazz, it doesn't have to love it; but say you love Madonna and it simply likes a few of her songs).
A2.2 Worst case scenario: everything else. How to fix it? Make a list of your likes and dislikes. Then make a list of your character's likes and dislikes. Again, don't make it your opposite. But if you love basketball, maybe its father loves basketball and it only has a passing interest. Or it prefers netball. It's perfectly ok to have the character share some of your interests, but change it a bit... and some is definitely less than 50%.
NOTE: If you have a particular hobby or interest, it's perfectly acceptable to maintain it. But don't let that hobby / interest dominate the story. Unless it is part of the plot, obviously. Like, you're a stamp collector and the character is obsessed with stamps. But do make sure it's a hobby/interest that helps the story move forward (or makes a statement about the character's personality) and doesn't simply show up because you think stamps are cool.
Q3. Is it perfect? Worse: is it cutely flawed?
A3.1 We all know a perfect character is irritating, to say the least, and must have flaws to be more human. But if the flaws are exagerated (it is so bad at cooking that entering a kitchen is a fire hazard) or cute (it is afraid of dogs and does the silliest things to keep them at bay), they are not real flaws. Talk to (or ask about) people that have that particular flaw. I can't cook, but it's basically because I don't like it so I do stuff with my mind elsewhere and try to take shortcuts that make the result less than appealing. And an actual fear of dogs is not cute; it's paralysing and mock-inducing (especially if kids / teenagers are involved, although adults can be just as bad).
A3.2 It has a serious flaw related with personality, say an apathic, brooding or emo behaviour. If it's exagerated, see above. If it only surfaces on some scenes... why? What triggers it? Is it to be cute or is it to excuse it of something? If, on the other hand, the behaviour is consistent... make sure those personality traits are there for a really good reason and it isn't just 'it has a really dark past that made it this way' because those personalities make cool characters. They don't. Cool characters may or may not have those traits, but the traits in themselves have no coolness factor associated to them.
A3.3 It has a real flaw that relates to the plot and a really good reason for it too (e.g. commitment phobic because of a traumatic experience involving his mother and a steady string of failed relationships). Is the character perfect in everything else? If no, cool; if yes, see below.
A3.4 Yes, it's perfect. How can I fix it? Whether it's a self-insert or just an original character that stole the author's heart, MCs can easily become perfect. First rule: make sure it isn't the best or the worst at anything. And if the plot requires it to be the best/worst... make it one of the best/worst, not the best/worst of the best/worst. And if it must be the best/worst at one thing, then make it just ONE thing. Or, show that the character believes it is the best/worst at lots of things when in reality it isn't that good/bad. Or, show the characters' lover acting as if it is perfect but then show it isn't so, and that the rest of the cast has no illusion either. And remember, would you like a person that is perfect in everything? Neither would the supporting characters. Maybe its lover loves it for its (alleged) perfectness, everyone else will hate it.
Q4. Does it suffer injustices that everyone ignores until the moment it explodes and then everyone feels bad because yes, life is so unfair for it?
A4.1 Yes. Wrong: so maybe its parents should have asked it direct questions instead of ransacking its room for proof of whatever. So maybe it decides to run away from home because of the unfairness of it all and because nobody understands it. But lets pause for a moment, shall we? What did the parents see? A teenager that refuses to explain what is the matter. A teenager who may have started taking drugs. Maybe they even asked direct questions... or asked them in the wrong way further pushing it away. Maybe they want to understand but, not getting any explanations from it, look at the 'scary' teen world and can only imagine the worst scenarios. Maybe ransacking its room was actually a call for help: 'tell us what is wrong!'
The thing is: what one thinks is unfair treatment may simply be the other party trying to help. Or maybe it wasn't unfair treatment at all; maybe it's just the consequences of poorly thought out actions.
Q5. Is the character 'magically' protected from consequences?
A5.1 Yes. No preferencial treatment can ever be allowed! Unless, of course, there is a biased headmaster, but then its colleagues will make it feel the consequences of being the headmaster's pet. If it gets in trouble, it must suffer through, even if that means breaking an arm or even losing a leg (How to train your dragon pulled it beautifully). Consequences are for everyone, whether it's catching a cold from not wearing a coat or getting shot from not wearing a vest.
A5.2 Only the little things, like... clumsily breaking a bunch of glasses in a shop? No preferencial treatment can ever be allowed! Make it pay for the glasses. If it says the wrong thing, don't let the others brush it off with a laugh and a frown; if it blundered, someone is going to be offended and it must apologise. No exceptions.
Q6. Does the character always get what it wants?
A6.1 Yes. Wrong: life doesn't always give us what we want no matter how much we work and sweat and hurt for it. Let it win some battles and lose others (meaningful battles, the others don't count). Let it win second prize rather than exactly what it wanted.
A6.2 Yes, but it really suffers for it. Wrong: I've been there. I had the typical sociopath yielding to its lover. Sure, she got pounded (figuratively or not) before she got what she wanted, but she still got it. And then I stopped and thought about it. Why would he yield? Why wouldn't he just kill her rather than put up with her? (tip: if psychos are involved, love is not a good answer; make sure there's more to it) Obviously, he would have killed her. It was a clear case of 'author protection'.
Bottom line: It's ok to have a special character, but make sure it isn't a SPECIAL one.