Раньше это были разные слова: "мiръ" (world) и "миръ" (peace). Но после орфографической реформы 1918 года буква "i" (а также буквы "Ѣ" и "Ѳ")были исключены из алфавита.

There are many such words in Russian.

One of the strangest coincidences is two different meanings of the word "брак":

1. "Брак" - marriage, wedlock. "Счастливый брак" ("Happy marriage").

2. "Брак" - defect; spoiled, substandard, defective articles of manufacture."Изделие с браком" ("a product with defect").

That's why in Russia often joke about the "marriage": <em>"Хорошее дело браком не назовут" ("A good thing never be called "брак").</em>

This is the beauty of all languages that a word can have more than one meaning.

If every word had just one meaning then we could have replaced a human translator or an interpreter with a machine a long time ago.

The simple answer "да" means "yes". The simple answer "нет" means "no". The simple answer "наверно" means "probably, may be". If one says "да, наверно", this means "probably yes, most likely yes". If one says "нет, наверно", this means "probably no, most likely no". Now you see that the positive particle "да" (="yes") has homonym "да" (=but, oh but, why, well, ...) which is to emphasize the following word. So "да нет!" means "but no!", "oh no!". And at last: "да нет, наверно" means "probably no", "most likely no".

There are many other homonyms of "да":

вы не торопитесь, да? = you're not in a hurry, are you?

да, еще одна новость = oh yes, (there's) another piece of news

вы знаете, он женился? - Да? А я и не знал = You know he has got married? - Has he? [Really / Indeed?] I didn't know

да будет свет! = let there be light!

шел он один, да еще и в темноте = he was walking alone and in the dark at that 

он охотно сделал бы это, да у него нет времени = he would be glad to do it, but he has no time for it 

and so on...

Soumyajit, why do you wonder? Homonyms are usual in any language. English has a lot of them too:

How much can a bare BEAR BEAR?

A LEAN cat could LEAN on a skunk!

Why don't we TOAST with a TOAST?

blind as a BAT; a baseball BAT; not to BAT an eyelid

a rock BAND; a rubber BAND

the BANK of the river; the BANK of England

a CUE to stop; billiard CUE

to tell a LIE; to LIE on the floor;

what do you MEAN? it was MEAN of him to say it; MEAN speed

the SOUND of music; a SOUND mind in a SOUND body

corn STALK; to STALK a deer

very WELL; a deep WELL

and so on...