Fix issue reported by Alterego:

Don't auto-add sub-boards to the area file if the echotag is already in the area file (but marked as pass-through or associated with a different sub-board already). This configuration could result in the same sub-board(s) being auto-added to the area file for every invocation of sbbsecho.

src/sbbs3/sbbsecho.c

@@ -1912,6 +1912,10 @@ bool add_sub_to_arealist(sub_t* sub, fidoaddr_t uplink)
 	}
 	strupr(echotag);
 
+	for(unsigned u=0; u < cfg.areas; u++) {
+		if(stricmp(cfg.area[u].tag, echotag) == 0)
+			return false;
+	}
 	lprintf(LOG_INFO, "Adding sub-board (%-*s) to Area %s with uplink %s, tag: %s"
 		,LEN_EXTCODE, sub->code, cfg.auto_add_to_areafile ? "File" : "List", smb_faddrtoa(&uplink, NULL), echotag);