user1434: If two programs both want to use the same bitcoin core instance to get notified about new blocks, do you add zmqpubhashblock=<address> twice to bitcoin.conf with two different port numbers, one for each program?
I have some "Meta" questions about this question too:
I suppose the answer to this question already exists somewhere and I spent some time trying to find it but failed.  What keywords would you use in a search engine to find the answer if it already exists online?  Is there a site where you'd expect this question to already have an answer, and if so, how would you find it?

user-1: 
do you add zmqpubhashblock= twice to bitcoin.conf with two different port numbers, one for each program?

Yes this is possible and mentioned in doc/zmq.md
The socket type is PUB and the address must be a valid ZeroMQ socket address. The same address can be used in more than one notification. The same notification can be specified more than once.

What keywords would you use in a search engine to find the answer if it already exists online?
Is there a site where you'd expect this question to already have an answer, and if so, how would you find it?

My approach for searching anything technical related to Bitcoin:
Add "Bitcoin Core" in things that are specific to Bitcoin Core. Copy whole error message if there is any. In your case it would be "zmqpubhashblock" bitcoin core
If search results in Google, DuckDuckGo etc. aren't helpful maybe search in different websites:

Bitcoin Stackexchange
Reddit
Bitcointalk
Github
Twitter

You can also search in Bitcoin Core IRC logs: https://bitcoin-irc.chaincode.com/bitcoin-core-dev/search?q=keyword

1434: <p>If two programs both want to use the same bitcoin core instance to get notified about new blocks, do you add <code>zmqpubhashblock=&lt;address&gt;</code> <em>twice</em> to bitcoin.conf with two different port numbers, one for each program?</p>
<p>I have some &quot;Meta&quot; questions about this question too:</p>
<p>I suppose the answer to this question already exists somewhere and I spent some time trying to find it but failed.  What keywords would you use in a search engine to find the answer if it already exists online?  Is there a site where you'd expect this question to already have an answer, and if so, how would you find it?</p>
 -1: <blockquote>
<p>do you add zmqpubhashblock= twice to bitcoin.conf with two different port numbers, one for each program?</p>
</blockquote>
<p>Yes this is possible and mentioned in <a href="https://github.com/bitcoin/bitcoin/blob/b05d3e76e7c72387b302cc21d420f31f384c8b10/doc/zmq.md" rel="nofollow noreferrer">doc/zmq.md</a></p>
<p><em>The socket type is PUB and the address must be a valid ZeroMQ socket address. The same address can be used in more than one notification. The same notification can be specified more than once.</em></p>
<blockquote>
<p>What keywords would you use in a search engine to find the answer if it already exists online?
Is there a site where you'd expect this question to already have an answer, and if so, how would you find it?</p>
</blockquote>
<p>My approach for searching anything technical related to Bitcoin:</p>
<p>Add &quot;Bitcoin Core&quot; in things that are specific to Bitcoin Core. Copy whole error message if there is any. In your case it would be <code>&quot;zmqpubhashblock&quot; bitcoin core</code></p>
<p>If search results in Google, DuckDuckGo etc. aren't helpful maybe search in different websites:</p>
<ol>
<li>Bitcoin Stackexchange</li>
<li>Reddit</li>
<li>Bitcointalk</li>
<li>Github</li>
<li>Twitter</li>
</ol>
<p>You can also search in Bitcoin Core IRC logs: <a href="https://bitcoin-irc.chaincode.com/bitcoin-core-dev/search?q=keyword" rel="nofollow noreferrer">https://bitcoin-irc.chaincode.com/bitcoin-core-dev/search?q=keyword</a></p>
