Retina, 19 bytes
Byte count assumes ISO 8859-1 encoding.
O`.+ Mm1`^(.+)¶\1 0
The input should be linefeed-separated. Output is
0 for falsy and
1 for truthy.
Try it online! (Slightly modified to support multiple space-separated test cases instead.)
Sort the lines in the input. If a prefix exists it will end up directly in front of the string which contains it.
Try to match (
M) a complete line which is also found at the beginning of the next line. The
m activates multiline mode such that
^ matches line beginnings and the
1 ensures that we only count at most one match such that the output is
1 in the result, we count the number of