Note: if you show all the code (how do you log in, how do you query the dataset), it will help us to help you.
Few very general advices for now.
The reason is becasue you select all (or at least more than one) records. When you do the output only once CF shows only first record from dataset. You can check this by looping over the dataset:
<cfloop query="Recordset1">
<cfoutput>#Recordset1.Username#</cfoutput><br />
</cfloop>
It should show all your records.
As Jason pointed you should select only single record of your user. When you perform login act, save user # (typically primary key, id) in Session scope (say, in Session.userid) and use it in queries later like this (I dont know your query, so this is just to show the idea):
<cfquery datasource="datasourceName" name="Recordset1">
SELECT Username FROM users WHERE id = <cfqueryparam cfsqltype="cf_sql_integer" value="#Session.userid#" />
</cfquery>
Supposing you have unique id s as PK, you ll get only one record in Recordset1, so your initial output will show correct username.