Hi Anna,
I have found a way to limit the Multiple Reply problem to only one Thread. The following will not reply to every thread in the Group, but will only reply to the one which receives a new email.

So I remain with a few issues:
1) What is the correct terminology? What is a conversation? Thread? Post? Group? Yes I have read the docs, but there is no clear definition. Please ELI5 if you can.
2) Variables: what is available? How do I extract and compare them? I found this (https://powerusers.microsoft.com/t5/Building-Flows/Stop-flow-adding-apply-to-each/m-p/142815#M14232) to stop the looping and I kind of get it, but I have not been successful in accessing any variables.
3) Going with the above, TimeStamps. Looking at the returned data from the "New Email Arrives to a Group" trigger, I get the following:
{"headers":{"Transfer-Encoding":"chunked","Strict-Transport-Security":"max-age=31536000","request-id":"f5e36ccc-98d1-480d-8499-849fe40109f4","client-request-id":"f5e36ccc-98d1-480d-8499-849fe40109f4","x-ms-ags-diagnostic":"{\"ServerInfo\":{\"DataCenter\":\"West US\",\"Slice\":\"E\",\"Ring\":\"5\",\"ScaleUnit\":\"002\",\"RoleInstance\":\"BY1PEPF00002593\"}}","Timing-Allow-Origin":"*","x-ms-apihub-cached-response":"true","retry-after":"60","Cache-Control":"private","Date":"Thu, 20 May 2021 01:18:50 GMT","Location":"https://unitedstates-002.azure-apim.net/apim/office365groupsmail/shared-office365grou-a38bb5fb-3acf-459b-88b7-8684-7572c825/trigger/v1.0/groups/6b6fd465-dc88-4383-a414-0c308f3e2cb6/conversations?%24select=id%2clastDeliveredDateTime&%24expand=threads(%24select%3did%3b%24expand%3dposts(%24select%3did%2ccreatedDateTime))&%24orderby=lastDeliveredDateTime+desc&NotFirstCall=true&TriggerState=AAMkADBjNTY3YTE5LWNlZjQtNDFmOS1iOTU0LTQ1ZDhmMWNmMDU1MQBGAAAAAABV6U8PpQVsSawyDJpdNznDBwDd0VYaAwk0TLOtrLG05FwjAAAAAAEMAADd0VYaAwk0TLOtrLG05FwjAAAn1PLJAAA%3D%2CAAMkADBjNTY3YTE5LWNlZjQtNDFmOS1iOTU0LTQ1ZDhmMWNmMDU1MQBGAAAAAABV6U8PpQVsSawyDJpdNznDBwDd0VYaAwk0TLOtrLG05FwjAAAAAAEMAADd0VYaAwk0TLOtrLG05FwjAAAn1PLIAAA%3D&%24filter=lastDeliveredDateTime%20gt%202021-05-20T01%3A16%3A34Z","Content-Type":"application/json; odata.metadata=minimal; odata.streaming=true; IEEE754Compatible=false; charset=utf-8","Content-Length":"796"},"body":{"id":"AAQkADBjNTY3YTE5LWNlZjQtNDFmOS1iOTU0LTQ1ZDhmMWNmMDU1MQAQAHVAm5n0lchDh8thPuNwhtI=","lastDeliveredDateTime":"2021-05-20T01:16:34Z","threads":[{"id":"AAQkADBjNTY3YTE5LWNlZjQtNDFmOS1iOTU0LTQ1ZDhmMWNmMDU1MQMkABAAdUCbmfSVyEOHy2E_43CG0hAAdUCbmfSVyEOHy2E_43CG0g==","posts":[{"@odata.etag":"W/\"CQAAABYAAADd0VYaAwk0TLOtrLG05FwjAAAn2Ijd\"","id":"AAMkADBjNTY3YTE5LWNlZjQtNDFmOS1iOTU0LTQ1ZDhmMWNmMDU1MQBGAAAAAABV6U8PpQVsSawyDJpdNznDBwDd0VYaAwk0TLOtrLG05FwjAAAAAAEMAADd0VYaAwk0TLOtrLG05FwjAAAn1PLJAAA=","createdDateTime":"2021-05-20T01:16:34Z","changeKey":"CQAAABYAAADd0VYaAwk0TLOtrLG05FwjAAAn2Ijd"}]}]}}
Looking at this information: https://docs.microsoft.com/en-us/connectors/office365groupsmail/ gives me hope, but is confusing as to how I can access some of it.
So bottom line: I am trying to create a flow that will send a reply email when a new "conversation" (to use the Outlook term) is started. If the "conversation" contains two or more emails then do not reply.
Thank you in advance for any insight that you can provide.