Merging lines

Hi,
I have a problem, hope someone can help
INPUT-file:

aaa;234;567;893
bbb;123;456;789
bbb;456;789;012
ccc;345;678;920
ccc;567;890;123
ccc;456;789;654

and I need an OUTPUT-file like this:

aaa;234;567;893
bbb;123;456;789;456;789;012
ccc;345;678;920;567;890;123;456;789;654

If the first field from current line matches the first field from next lines
, then merge the lines to one line.

Thanks a lot