plenplena

A doubled root, which is a device Esperanto uses sparingly and effectively. Repeat a root and you intensify it: plena is full, plenplena is full to the brim, stuffed, chock-a-block. It is often written plen-plena with a hyphen, and both spellings are correct.

Is it the only one? No, and it would be a nicer story if it were. Two other genuine reduplications are already in this course, and both are adverbs: finfine at rank 1012, meaning finally, at long last, and fojfoje at 1282, meaning occasionally. Ununura at 1620 is a near miss of the same shape, made from unu plus nura rather than from one root twice. So the device exists, it is productive enough that you could coin belbela and be understood, and it stays rare enough that the three words above are most of what you will meet.

And the words that only look doubled. Popolo, purpura, murmuri, barbaro and papago all repeat a syllable and none of them is a reduplication: they are single borrowed roots that happened to arrive with that shape, from Latin populus, purpura, murmurare, barbarus and the parrot's own name. Splitting papago into pap plus ago produces a priest's action, which is not a bird.

The distinction is worth having straight, because the doubling in plenplena carries meaning and the doubling in murmuri carries none.