Use abs (negative radii are allowed)

Author: ivankokan

base/geometry.asy

@@ -6318,7 +6318,7 @@ point radicalcenter(circle c1, circle c2, bool abort=true)
    having the same power of a point with respect to both circles).</documentation></function></asyxml>*/
   // TODO Consider degenerate circle(s)
   if (c1.C == c2.C) {
-    if (c1.r == c2.r && !abort) {
+    if (abs(c1.r) == abs(c2.r) && !abort) {
       warning("radicalcenter", "The common center is returned as the most convenient point
 for two circles which are both concentric and congruent.");
       return c1.C;