[Plugin: Yoast WordPress SEO] duplicate meta tags appearing in page source

We have just set up a new website and installed the Yoast WordPress SEO plugin. First time we have used it.

We just noticed that in our page source (of any page on the website), each page has two meta titles and two meta descriptions. For example here is part of the html from one of our pages:

<!– This site is optimized with the Yoast WordPress SEO plugin v1.4.24 – http://yoast.com/wordpress/seo/ –>
<title>For sake of support case, let’s assume this is the actual title</title>
<meta name=”description” content=”For the sake of this support case, let’s pretend that this is the actual meta description.”/>
<link rel=”canonical” href=”http://www&#8230;..” />
<meta property=”og:locale” content=”en_US” />
<meta property=”og:type” content=”article” />
<meta property=”og:title” content=”For sake of support case, let’s assume this is the actual title” />
<meta property=”og:description” content=”For the sake of this support case, let’s pretend that this is the actual meta description.” />
<meta property=”og:url” content=”http://www&#8230;..” />
<meta property=”og:site_name” content=”…..” />
<meta property=”article:published_time” content=”2013-10-22T10:25:02+00:00″ />
<meta property=”article:modified_time” content=”2014-02-14T11:21:05+00:00″ />
<meta property=”og:image” content=”http://www…&#8230;.com/wp-content/uploads/2014/02/coffee-cups.png” />
<!– / Yoast WordPress SEO plugin. –>

As you can see we have 4 tags:
<title>
<meta name=”description”
<meta property=”og:title”
<meta property=”og:description”

The first two look very normal to me but the other two look strange (“og”).

Also, on each page the two titles have the exact same content. Likewise for the two descriptions. It’s not like one is coming from the yoast plugin and another is coming from the a standard title/description in the theme.

Any ideas? thanks!