What should this print?

echo -e 'aa\naa\naa\n' | uniq -d

To me this says:

1. uniqueness is defined by whole line so there is 1 unique value 'aa';
2. -d option say to 'only print duplicate lines';
3. 1st 'aa' is (so far) unique so it should NOT be printed;
4. 2nd 'aa' is not unique so it SHOULD be printed; and
5. 3rd 'aa' is not unique so it SHOULD also be printed.

I think I should get this:

aa
aa

But I get this:

aa

To see what duplicated line is printed I tried this:

echo -e 'a1\na2\na3\na4\n' | uniq -d -w 1
a1

So, first line is printed. This is not what I expected at all.



Now, -D means 'print all duplicate lines' and

echo -e 'aa\naa\naa\n' | uniq -D

prints what I expect it to:

aa
aa
aa

Now, -D and -u means 'print all duplicate lines' and 'only print unique
lines'.

I think this should print all lines since union of all unique lines and all
duplicate lines is all lines.

But,

echo -e 'aa\naa\naa\n' | uniq -Du

prints this:

aa
aa

To see what lines are being printed I tried this:

echo -e 'a1\na2\na3\na4\n' | uniq -Du -w 1
a1
a2
a3

Therefore -Du prints first N-1 matching lines and not last matching line.

(Which is sort-of like what I expect -d to print)

Are these bugs?