redir: Do not count multiple rules with if statements as duplicates

This allows you to have multiple redir directives conditioned solely
upon if statements, without regard to path.

caddyhttp/httpserver/condition.go

@@ -40,6 +40,7 @@ func SetupIfMatcher(controller *caddy.Controller) (RequestMatcher, error) {
 				return matcher, err
 			}
 			matcher.ifs = append(matcher.ifs, ifc)
+			matcher.Enabled = true
 		case "if_op":
 			if !c.NextArg() {
 				return matcher, c.ArgErr()
@@ -155,6 +156,7 @@ func (i ifCond) True(r *http.Request) bool {
 
 // IfMatcher is a RequestMatcher for 'if' conditions.
 type IfMatcher struct {
+	Enabled bool     // if true, matcher has been configured; otherwise it's no-op
 	ifs     []ifCond // list of If
 	isOr    bool     // if true, conditions are 'or' instead of 'and'
 }

caddyhttp/redirect/setup.go

@@ -101,11 +101,14 @@ func redirParse(c *caddy.Controller) ([]Rule, error) {
 			return c.Err("'from' and 'to' values of redirect rule cannot be the same")
 		}
 
+		// prevent obvious duplicates (rules with if statements exempt)
+		if ifm, ok := rule.RequestMatcher.(httpserver.IfMatcher); !ok || !ifm.Enabled {
 			for _, otherRule := range redirects {
 				if otherRule.FromPath == rule.FromPath {
 					return c.Errf("rule with duplicate 'from' value: %s -> %s", otherRule.FromPath, otherRule.To)
 				}
 			}
+		}
 
 		redirects = append(redirects, rule)
 		return nil