doppelt
double
Indicating twice the amount or number of something.
Für das Rezept brauche ich die doppelte Menge Mehl.
For the recipe, I need double the amount of flour.

double
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double
Indicating twice the amount or number of something.
Für das Rezept brauche ich die doppelte Menge Mehl.
For the recipe, I need double the amount of flour.

The word 'doppelt' (double/twice) can be used like an adverb without a declension ending to modify a verb or another adjective. It then means 'twice' or 'in a double way'.
Ich habe die Tür doppelt abgeschlossen.
I double-locked the door.
'Doppelt' (double/twice) is very often used in comparisons with the structure 'doppelt so + Adjektiv + wie' (twice as + adjective + as). This expresses that something has twice as much of a quality as something else.
Sein neues Auto ist doppelt so teuer wie mein altes.
His new car is twice as expensive as my old one.
In a figurative sense, 'doppelt' (double) can also mean 'dishonest' or 'ambiguous'. It often occurs in fixed expressions like 'ein doppeltes Spiel spielen' (to play a double game) or 'ein doppeltes Leben führen' (to lead a double life).
Der Agent führte ein gefährliches doppeltes Leben.
The agent led a dangerous double life.
The expression 'Doppelt hält besser' (better safe than sorry) is a well-known proverb. It is used to express that an additional safeguard or repeating an action increases security.
Ich habe eine Kopie der wichtigen Dokumente gemacht – doppelt hält besser.
I made a copy of the important documents — better safe than sorry.
simple / just
'einfach' and 'doppelt' are opposites when it is about a single or double version of something. 'doppelt' means: present twice, twice as much, or made of two parts. In certain contexts, 'einfach' can mean 'only once' or 'one-way', especially with tickets. The words can be confused because 'einfach' very often also means 'easy'. In that sense, however, 'doppelt' is not the opposite. The contrast only fits when something is being counted: once or twice.

Bitte prüft die Adresse doppelt, damit das Paket nicht falsch ankommt.
Please double-check the address so the package does not arrive at the wrong place.
Am Schalter bestelle ich eine einfache Fahrt nach Köln.
At the counter, I order a one-way trip to Cologne.