Droping A CONSTRAINT

I am having problem to find the right syntax to DROP a column with contrainst and recrate it
I get an error

if exists ( select * from INFORMATION_SCHEMA.COLUMNS
where TABLE_NAME='myTable'
and COLUMN_NAME='myDate' )
ALTER TABLE [dbo].[myTable] DROP COLUMN myDate
GO

ALTER TABLE [dbo].[myTable] WITH NOCHECK ADD
myDate datetime CONSTRAINT [DF_myDate] DEFAULT (GetDate())
GO

Query Analyser says :
Server: Msg 5074, Level 16, State 1, Line 5
The object 'DF_myDate' is dependent on column 'myDate'.
Server: Msg 4922, Level 16, State 1, Line 5
ALTER TABLE DROP COLUMN myDate failed because one or more objects access this column.
Server: Msg 2705, Level 16, State 4, Line 2
Column names in each table must be unique. Column name 'myDate' in table 'dbo.myTable' is specified more than once.

thank you for helping