I try to avoid this site but this page came up when I was searching for Kālidāsa on StackExchange, so let me post an answer anyway.
Firstly, this is obviously not from Kālidāsa's Meghadūta. The Meghadūta is entirely written in the Mandākrāntā metre, which has 17 syllables to each pāda (68 total). This one here is a standard śloka (anuṣṭup), with 8 syllables in each pāda (32 total).
It also does not occur in any of the other works of Kālidāsa, and is just not his style.
Instead, it is a subhāṣita, a pithy saying or proverb. Such subhāṣitas occur in many anthologies and tend to get attracted into editions of works like the Hitopadeśa or Pañcatantra. (Look at these works, or Bhartṛhari's Nīti-śataka, for many similar verses about the world, about bad people, about fools, etc.)
It is a nice śloka with a clever use of the same words to mean different things:
मनस्येकं वचस्येकं कर्मण्येकं दुरात्मनाम् ।
मनस्येकं वचस्येकं कर्मण्येकं महात्मनाम् ॥
manasyekaṃ vacasyekaṃ karmaṇyekaṃ durātmanām /
manasyekaṃ vacasyekaṃ karmaṇyekaṃ mahātmanām //
With the sandhis split:
मनसि एकं वचसि एकं कर्मणि एकं दुर्-आत्मनाम् /
मनसि एकं वचसि एकं कर्मणि एकं महा-आत्मनाम् //
manasi ekaṃ vacasi ekaṃ karmaṇi ekaṃ dur-ātmanām /
manasi ekaṃ vacasi ekaṃ karmaṇi ekaṃ mahā-ātmanām //
Literal translation / gloss:
[in mind] [one] [in speech] [one] [in action] [one] [of bad people] /
[in mind] [one] [in speech] [one] [in action] [one] [of great people] //
The two sentences are grammatically equivalent and there's no trickery with the words; it's just based on the context/semantics that we are led to understand them differently.
The former is the colloquial sense as in "one here, one there…" (we understand that they are different). The latter uses the repetition of "one" for emphasis: in thought, word and deed, the great have only one thing.
(Some sources change the दुरात्मनाम् line to "मनस्यन्यद् वचस्यन्यत् कर्मण्यन्यद् दुरात्मनाम्" but that's just killing the nice thing about the verse.)
Finally, as a proverb, it's not something you should think too hard about, or treat as a law of nature. Famous proverbs like "An apple a day keeps the doctor away" or "Time is money" are too easy to pick holes in; yet they express some real truth, appropriate in some contexts.
Enjoy the common-sense idea couched in memorable language.