Удалено col-sm-12
форма form-group
и удалено margin
из последних form-group
<div class="form-group researchPapers">
<div class="input-group">
<div class="form-group">
<input type="text" id="title[]" name="title[]" class="form-control" placeholder="Title of Paper" />
</div>
<div class="form-group">
<input type="text" id="journal[]" name="journal[]" class="form-control" placeholder="Name of Journal" />
</div>
<div class="input-group-addon">
<a href="javascript:void(0)" class="btn btn-success addMore"><span class="glyphicon glyphicon glyphicon-plus" aria-hidden="true"></span> Add New</a>
</div>
</div>
</div>
CSS
.input-group .form-group:nth-child(2) {
margin-bottom: 0;
}
Для общего представления о множественных входах (используйте bootstrap 4 класса для гибкого, я даю общее представление)
<div class="form-group">
<input type="text" id="journal[]" name="journal[]" class="form-control" placeholder="Name of Journal" style="
flex-basis: 1;
"><input type="text" id="journal[]" name="journal[]" class="form-control" placeholder="Name of Journal"><input type="text" id="journal[]" name="journal[]" class="form-control" placeholder="Name of Journal"><input type="text" id="journal[]" name="journal[]" class="form-control" placeholder="Name of Journal">
</div>
CSS
.input-group .form-group:nth-child(2) {
margin-bottom: 0;
display: flex;
}
.input-group .form-group:nth-child(2) input + input {
margin-left: 15px;
}