How can I reproduce it? The following works fine (I get "0.1")
var wb = new XLWorkbook();
var ws = wb.AddWorksheet("Sheet1");
ws.Cell("A1").SetValue(0.09).Style.NumberFormat.SetFormat("0.0");
var s = ws.Cell("A1").GetFormattedString();
btw, how did you get 0.10000000000000009 into Excel? The most I can put in is 0.100000000000009