CJam, 26 bytes
LLq{+_a2$&{a+L}|}/:X+X!S**
Test it here.
Explanation
L e# Push an empty array to keep track if the previous segments.
L e# Push an empty array to build the current segment.
q{ e# For each character in the input...
+ e# Add it to the current segment.
_a2$& e# Duplicate and check if it's already in the segment list.
{ e# If not...
a+L e# Add it to the list and push a new empty array for the next segment.
}|
}/
:X+ e# Store the trailing segment in X and add it's *characters* to the list.
e# For valid splittings, this trailing segment will be empty, so that the
e# list remains unchanged.
X! e# Push X again and take the logical NOT.
S* e# Get that many spaces, i.e. 1 for valid segments and 0 otherwise.
* e# Join the list with this string between elements.
