problem while adding a column

When ever trying to add a new column to the table get the following error

ERROR:  duplicate key violates unique constraint
"pg_attribute_relid_attnum_index"

While the new column that is added is not there in the table before.

Doing a select * from table from the same tables also giving an error saying:

ERROR:  cache lookup failed for type 0

Any idea what could be going wrong here??

/Shoaib