We may not be as unique as we think
The big sigh when we compose something we're convinced must be totally unique, only to stumble across its already-published cousin a while later. Or vice versa: after we've published our special poem, another one from someone else follows with a hauntingly similar treatment. Possibly the "original" imagery is very similar or the particular message or concept or sometimes a few key words—or some combination of those things. The shorter the poem—such as in the case of tanka and, even more so, haiku—the greater the inevitability that sort of thing is going to happen.
Perhaps we should enjoy the synchronicity. Perhaps we also should find solace when we know we're writing who we are, infusing ourselves and our experiences as deeply as possible into our words, delivering our own takes on what has "already been written," even if we may be unaware at the time that it has.
Margaret Dornaus sums up this situation well in a post, "Revisiting déjà-ku . . .," at her Haiku-doodle blog. So I won't say anything more here and will let you read her beautiful words.