Here's one I get a lot:
"Other people have already written about this. I guess I'd better write about something else."
People have been writing about the same topics for centuries – love, hate, desire, relationships, crime, food, birth, death, deception, dogs, aliens, friends, enemies, you name it (okay, maybe not cell phones, but you get the idea). If writers were to stop writing simply because someone else beat them to the topic; well, most every bookshelf would be empty, magazine or newspaper pages and websites would be blank.
The point is that what you write will be different because you are a different writer from all the others who have already mined the same material. Or at least, you'd better be. If on the other hand, you have nothing new to say, if the only way you can manage to write about X, if the only thing you have to contribute about X simply mirrors what others have already done, then by all means, stay away from X and write about Y instead.
If abandoning your chosen topic is more about marketplace concerns, then I have even better news: Forget it. If you make choices about what to write based on what's selling at bookstores TODAY, you will be in for a lifetime of disappointment. Let's say you notice that books about twins are hot, and you are working on a novel or memoir about twins, you'd be mistaken to decide either that you'll be too late to the party by the time your manuscript is done OR that you if you write really fast, you can get in on a hot trend.
Either may turn out to be true, or neither – like so many other once-hot subjects, the topic may slip from its pinnacle, but remain a healthy but smaller part of the book market anyway. The thing is, you won't know while you are still writing which scenario will play out, and anyway, do you really want to determine what you write about based on something as fickle as readers' tastes? I'm not being naïve; I realize that writing a new vampire or wizard series is probably not going to generate huge interest right now. Except -- I could be wrong. (On the other hand, if you can reliably predict what will be hot in a literary sense, say, two to three years down the line, then I take it all back. And, can I get your number?)
This goes for short pieces of writing, too. So many writers have seen an essay or feature in a particular media venue – usually a publication or site they greatly admire and aspire to being published in, with a huge circulation, top freelancer pay rates and mucho prestige – and thrown up their arms. "Ugh. So-and-So just wrote a column about X. So I guess it's been done. "
Who cares if So-and-So wrote about it? You know what? It may be a really good thing that So-and-So raised awareness about X, introduced X into the conversation, put other editors on alert that there's something to be said about X. Now you can approach editors at venues other than the admired, highly valued one, high paying, prestigious one you aspire to, and concentrate on those that – let's face it – are probably more likely to publish your work.
And if that isn't what you want to hear, then there's this: Next time, stop dawdling over your pretty-damned-good essay or manuscript or query and hit send, instead of worrying over every comma (for the 14th blessed time) and talking yourself into the idea that it's not going to fly anyway, so why submit it? Be first next time. But if you can't, being second, tenth, or 203rd is okay too. It's a pretty big literary world out there and chances are very good you can find a place for your work where what you have to say about X will be fresh, and first, for them.
Meanwhile, you could also just stop reading so much! (kidding, I think)
Note: Your can read the rest of the Stuff My Writing Students Say series here.