gdb,why "next" show each source line twice?

everyone,when use "next" instruction within gdb,i found each line of sourcecode display twice-----i am quite sure,these code are not in any loop.this is the phenomenon:

(gdb) frame
#0  ap_get_client_block (r=0x8560d48, 
    buffer=0xb68501b7 "<?xml version=\"1.0\" encoding=\"utf-8\"?><Root><OperCode>SMS101</OperCode><AppId>SMSMsgFilterReq</AppId><Req><UserMobile>13925237429</UserMobile><SendMsg>abc圲34¨圲23a露07\214圲21￠＜237朲11?东215圲10°＜214朲27￥応227露07\214正常,hao"..., bufsiz=81920) at http_filters.c:1540
1540        if (r->remaining < 0 || (!r->read_chunked && r->remaining == 0)) {
(gdb) n
1544        bb = apr_brigade_create(r->pool, r->connection->bucket_alloc);
(gdb) 
1545        if (bb == NULL) {
(gdb) 
1544        bb = apr_brigade_create(r->pool, r->connection->bucket_alloc);
(gdb) 
1545        if (bb == NULL) {

-----the line 1544,1545 are not in any loop,but they got repeated. May anybody enlighten me?