Why does my find command get executed two times?

You have a race condition - first find finds ./a and copies it to test/a, then it finds the newly copied ./test/a and tries to copy it again:

$ find . -mtime -1 -name a -print -exec cp -v {} test/ ';'
./a
'./a' -> 'test/a'
./test/a
cp: './test/a' and 'test/a' are the same file

You could avoid that by telling find not to descend into the target directory ex.

find . -path ./test -prune -o -mtime -1 -name a -exec cp {} test/ ';'

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