Is there a bug with Netscape 4.x's <layer> or <ilayer> tag?? Please go to the following url using Netscape 4.x and see if the "included text" is repeated twice? The one in the <td> of the <table> is expected, but why there is another one outside of the <table> ?
ilayer is a Netscape 4.x proprietary thing iframe is the HTML standard, and is supported in Netscape 6, plus Explorer and Opera.
One idea is to nest the two sets of tags. Putting the iframe tags INSIDE the ilayer tags is reported to be the best way. Any external source file should be a well-formed HTML document (HEAD and BODY tags, and it validates) and not merely an HTML fragment. That's my best guess for why you're getting the duplicated text -- your included file is just a table and not a complete HTML page.
OK, I think I've got it. In your included file, layer_inc.htm, take the text out of the table. Since you are already writing a table in the main file, you don't need a second one. The included code can be like this: <html> <head><title>dummy</title></head> <body> Included text </body> </html>