SQL Insert Query to prevent duplicate data

I would recommend you have a primary key (or setup a unique constraint) in your table.
This would prevent duplicate entry.

How to apply UNIQUE constraints to existing SQL columns[^] might help you get started.

Since you did not
1) State type of SQL (MySQL, Oracle, SQL Server....)
2) Give example of what you have tried
I will give you a hint for SQL Server. Look at LEFT JOIN

INSERT INTO Table2 (variable list)
SELECT variable list 
FROM Table1 
LEFT JOIN Table2 
ON Table1.keyfield = Table2.keyfield 
WHERE Table2.keyfield IS NULL;

create Proc Demo

@Empid int ,
@Name nvarchar(50),
@Phone nvarchar (50)
As
if not exist (select * from EMP where Empid=@Empid)
Begin
insert into EMP(Empid,Name,Phone) values (@Empid,@Name,@Phone)
End

sp ---

declare a var A

select count(*) into A from table where id=@id
if(A>0)
//insert
else
//update

end if