title: RxJs how to merge two overlapping observable into one
body: <p>I have two observables:</p>
  
  <pre><code>-1-2-3-4-5-6-7-8-9-10-11-12-13-14-15-|
  -13--14--15--16--17--18--19-----20---------21--------------22------23--24--&gt;
  </code></pre>
  
  <p>The first contains some increasing number, but stops after a while (these are the cursor result from the database)
  The second are continuously emitting increasing number. Contains some number from the first, but don't stop emitting. (These are the newly inserted data to the database)</p>
  
  <p>I want this two observable to look one continuous observable like this:</p>
  
  <pre><code>-1-2-3-4-5-6-7-8-9-10-11-12-13-14-15-16-17-18-19-20-21-----22------23--24--&gt;
  </code></pre>
  
  <p>This observable contains every number only once, keeping the emission order.</p>
  
  <p>How can it be solved with using as less memory as possible?</p>