Santiago Balaguer García wrote:

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The answer is not totally correct. Because a microcuts in the connectibity of hotspot cause that hotspot re-sends the acct request.

No, because then the Acct-Session-Id would remain the same.

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In that case, you have to desactivate: - accounting_start_query_alt - accounting_stop_query_alt

No. Re-read the original post. The Acct-Session-Id is different, hence 2 sessions are being inserted into SQL, but no Accounting-Stop ever gets send for the 1st.

The NAS is misbehaving.

The problem was in the NAS itself indeed and not in freeradius. Thanks.