Dell Inspiron 15R 5520 :: Few Specific Characters Appear Only After Typing Key Twice

I'm facing a very strange issue in my Inspiron 15R 5520 which I bought in August 2012. There are few characters, which appear only after typing the corresponding key on the keypad twice.

e.g. If I want to type tilde(~) character, then I have to press the key on the lest side of Numeric key 1. But, if I press it once, nothing appears on the screen. And If I press this key twice, then I get two tilde (~) characters. Same behaviour happens for four other characters. Given below is the list of characters, which do not show up by pressing key once, rather twice.

~
`
^
'
"

Surprisingly, when I was typing keys Shift+6 in the compose area of this forum, then the character caret (^) appeared after pressing key once. Whereas, if I try it in Notepad or Word Pad or in the address bar of Mozila FireFox, I'll have to press the key twice so that character appears twice and then delete the extra character...