string text = "<p>test</p><p>test</p><p>test</p><p>test</p><p>test</p>";
foreach(string s in Regex.Matches(text, @"</.*?>").Cast<Match>().Select(s => s.Value).Distinct()){
text = text.Replace(s, string.Format("{0}{1}",s,Environment.NewLine));
// text = text.Replace(s, string.Join(string.Empty,new[]{s,Environment.NewLine})); // or
// text = text.Replace(s, s+Environment.NewLine); // or
}
return text;