HTML/JQuery: Button submits once when it shouldn't at all

HTML/JQuery: Button submits once when it shouldn't at all

I'm using jQuery to allow the form to submit, so I don't want the form to
submit on it's own.
Here's my form:
<form class="navbar-form navbar-right" id="signform">
<div class="form-group">
<input type="text" placeholder="Username" class="form-control">
</div>
<div class="form-group">
<input type="password" placeholder="Password" class="form-control">
</div>
<button id="signbtn" class="btn btn-success">Sign in</button>
</form>
Here's my JQuery code:
var signingIn = false;
$(document).ready(function() {
$('#signbtn').click(function() {
if (signingIn) return;
signingIn = true;
var img = $('<img id="loader" style="padding-left: 10px;"
src="loader.gif"/>');
$(this).after(img);
var data = $('signform').serialize();
$.ajax({
url: '/logs',
type: 'post',
data: data,
done: function() {
$('#loader').remove();
alert('success');
}
});
});
});
Now when I go to the url: website.com/index.html and click Sign in, it
refreshes the page (what I don't want at all) and goes to
website.com/index.html? - Now when I click it again, nothing happens
(exactly what I want all the time so my added elements through jQuery
isn't lost).