Why “suricata -T” command show me below error:

<Warning> - [ERRCODE: SC_ERR_DUPLICATE_SIG(176)] - Signature with newer revision, so the older sig replaced by this new signature "alert ip [,,,,,,,,,,,,,,,,,,,] any -> $HOME_NET any (msg:"ET DROP Dshield Block Listed Source group 1"; reference:url,feeds.dshield.org/block.txt; threshold: type limit, track by_src, seconds 3600, count 1; classtype:misc-attack; flowbits:set,ET.Evil; flowbits:set,ET.DshieldIP; sid:2402000; rev:5689; metadata:affected_product Any, attack_target Any, deployment Perimeter, tag Dshield, signature_severity Major, created_at 2010_12_30, updated_at 2020_10_05;)"

Can I clear Suricata-IDS cache?

Thank you.

That doesn’t have to do anything with a cache. You have the same signature twice but with different revision, so the newer revision will be used. You might want to remove the old revision one.

What is the easiest way to find the old version?

Grep for the SID in your rule files and compare the rev number.