Is there a reason that there's a dagesh in the first nun?

Indeed, there is:
Actually, there should have been two nun's here, and they merged - so, for added clarity, I usually add a dagesh whenever I can in such cases.

Another such case:
הוא הבחין, אנחנו הבחנּו
Actually, the dagesh is not a must, but it does help beginners in Hebrew, I guess, at it shows that the nun is supposed to be doubled.

One nun belongs to the root of the verb, the other is dictated by the conjugation.

I should add that it should have been written as follows:
הכננו
but as we cannot have this kind of combination (as the first nun is with a schwa and the second is vocalized), the two nuns merge into one.
In this case I prefer to add dagesh (almost always, unless I don't have an option of adding nikkud).

I see. Is the nun doubled in pronunciation?

Speaking as a non-native Hebrew speaker, I found the dagesh puzzling because there were no other niqudot in the sentence. Would a newspaper or book for native adult speakers ever insert a dagesh in such a place?

Thanks,
Alan

No, a newspaper or a book wouldn't add it. Well, a book might add it, as books usually have partial nikkud (or at least printing usually allows this option); as for newspapers, I don't recall having seen nikkud there.

I believe the nun should be somewhat emphasized, as if doubled, but I guess Hebrew speakers don't really (or don't always) pronounce it this way. :)

Tell me something, Alan:
If I post the same sentence, yet without the dagesh - will the Tatoeba system regard it as a duplicate and delete it automatically (when running the duplicate deletion session)?

If not, I guess I can add the other variant, with no nikkud at all.

I doubt that Tatoeba would regard it as a duplicate, since that would require pretty sophisticated (and perhaps slow) language-specific algorithms. However, I can't say for certain. sysko would know.

I believe that I've seen a dagesh in surprising places and never understood why. (It didn't seem to be in a proper noun, for instance, which I would understand.) One example was in lyrics to a particular song on the Internet. I wouldn't know how to find it again, though.

If you come across any strange dagesh, anywhere, feel free to ask.

OK, then, regarding the same variant without a dagesh - I'll post it. In case it should be deleted by the system, I guess it will automatically be deleted in due time.