Tcl, 28 bytes

puts [split [join $s ""] ""]

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Haskell, 22 bytes

(((:" ")=<<)=<<).words

Laikoni saved 2 bytes with a nice use of words. Previous answer:

(=<<)(:" ").filter(>' ')

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Pyth, 4 bytes

jdsc

Splitting on whitespace and then joining seems shorter than removing the spaces.

In pseudocode:

'     ' d,Q = " ",input()  # preinitialized variables
'jd   ' d.join(
'  s  '     sum(
'   cQ'         Q.split() ))

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Retina, 8

 

.
$0  

Whitespace is significant - there are single spaces at the end of the first and last lines.

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V, 7 bytes

Íó*/ 
X

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Just a straightforward regex, and an "X" to delete one leading space.

Jelly, 3 bytes

ḟ⁶K

ḟilter out spaces (⁶) then join by space (K).

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Python, 36 bytes

lambda s:" ".join(s.replace(" ",""))