S. R. Faruqi:

jahāñ jahāñ = everywhere, extremely much

An excellent example of 'iham of sound' [īhām-e ṣaut] is in this verse, because in the first line jahāñ jahāñ seems apparently to be a repetition of an Urdu relative pronoun, but in reality it's Persian and means 'in every place'. Ghalib has, in Persian, used it like this: [an example in which jahāñ jahāñ clearly means 'everywhere'].

In Mir's verse the 'tajnis' among abr , ab , āb is fine. Another aspect of the meaning is also excellent: that when tears would fill the eyes, then everything appears to be drowned in water. A further point is that by using the ambiguous structure jāve naz̤ar jahāñ tak he has also suggested that in the eyes of all humankind there's nothing but water, with the sole exception of himself.

It's possible that Mir might have used jahāñ jahāñ like yak jahāñ , with the meaning of 'extremely much'. Testimony for this meaning is given by the [Persian dictionary] farhang-e ānand rāj .

FWP:

SETS == REPETITION
MOTIFS
NAMES
TERMS == IHAM; TAJNIS

Thanks to the definition of jahāñ jahāñ provided by SRF (and not available in Platts or Steingass), we can take it as an example of iham. For the jahāñ jahāñ looks perfectly plausible as a distributive use of the relative pronoun ('wherever, in all the places where'); it makes us expect in the second line something like 'in all those places bloody flowers sprang up', or something of that ilk. But then the second line confronts us simply with water water everywhere (literally, with enjoyable wordplay, 'from head to head'), so we are perplexed to explain the jahāñ jahāñ . Only by going back and mentally reinterpreting the first line (if we know the rare Persianized meaning of jahāñ jahāñ ) can we fully understand the verse, and savor the three-fold jahāñ wordplay.

Moral: even using a good dictionary might not be enough for effective reading of Mir's ghazal verses; but not using a good dictionary will almost guarantee an inability to really fathom the verse. We are lucky that SRF's commentary offers us, among many other benefits, the fruits of his extensive dictionary work.