words - The use of ただ and だけ together redundant?

ただ一つだけ守りたいものを最後まで守り通せばいい。

This quote comes from episode 9 of the anime Mahou Shoujo Madoka Magica and the English subtitles translate it as "Protect the thing you want to protect until the very end."

However, ただ means "only" and だけ means... "only." Why did the character say "only one thing only"? Wouldn't either ただ or だけ have been sufficient alone?

Answer

It's not redundant to use ただ (or たった etc.) and だけ (or ばかり, のみ etc.) together. It's perfectly natural to say, for example:

「ただ広いだけの庭」

「ただ一人だけ生き残る」
「ただ笑うばかりだ」
「たった一度会っただけだ」
(Examples taken from 明鏡国語辞典)

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ただ一つだけ守りたいものを最後まで守り通せばいい。
Wouldn't either ただ or だけ have been sufficient alone?

Yes, you could rephrase your sentence like this, without changing the meaning:

ただ一つ、守りたいものを最後まで守り通せばいい。
一つだけ守りたいものを最後まで守り通せばいい。

But ただ～～だけ would sound more emphatic (and could sound a bit more emotional, too).