avformat/assdec: do not drop duplicate subtitles

Duplicates are perfectly legitimate with this format. A correct
renderer will typically show them stacked.

libavformat/assdec.c

@@ -125,6 +125,8 @@ static int ass_read_header(AVFormatContext *s)
     av_bprint_init(&line,   0, AV_BPRINT_SIZE_UNLIMITED);
     av_bprint_init(&rline,  0, AV_BPRINT_SIZE_UNLIMITED);
 
+    ass->q.keep_duplicates = 1;
+
     for (;;) {
         int64_t pos = get_line(&line, &tr);
         int64_t ts_start = AV_NOPTS_VALUE;