In the .NET framework, there are two "TextBox" classes:
They don't conflict because they are in two different namespaces. Think of your question in that regard. Yes, you have a "Banner1" in two places however, they are in different places in a control hierarchy. One is in the master page's controls and one in the page's content placeholder. If you look at the HTML that is emitted to the browser for the aboutus.aspx page
, you'll see two distinct controls names:
ctl00_cpMainContent_Banner1 - This is the horizontal banner in the page's content.
ctl00_Banner1 - This is the one that lives in the master page.
Note: Unless explicitly named, the top level master page control will typically be "ctl00".